There is nothing wrong with offering du‘aa’ together after a lecture
Question

                Is it permissible to offer du‘aa’ together, such as if the imam offers du‘aa’ after giving a lecture, for example?
Praise be to Allah.
    |Du‘aa’ is one of the best acts of worship by means of which 
    the Muslim may worship his Lord. Allah, may He be exalted, says 
    (interpretation of the meaning):

    “And your Lord 
    said: ‘Invoke Me, (i.e. believe in My Oneness (Islamic Monotheism)) (and ask 
    Me for anything) I will respond to your (invocation). Verily! Those who 
    scorn My worship (i.e. do not invoke Me, and do not believe in My Oneness, 
    (Islamic Monotheism)) they will surely enter Hell in humiliation!’”

    [Ghaafir 40:60]. 

    It was narrated from an-Nu‘maan ibn Basheer, that the Prophet 
    (blessings and peace of Allah be upon him) said: “Du‘aa’ is worship. Your 
    Lord said: ‘Invoke 
    Me, I will respond to your (invocation).’” 
    Narrated and classed as saheeh by at-Tirmidhi, 2969; also narrated by Abu 
    Dawood, 1479; Ibn Maajah, 3828; classed as saheeh by al-Albaani in Saheeh 
    Abi Dawood. 

    Here we should point out an important matter about which many 
    people are confused, namely the difference between “making dhikr together” 
    and “offering du‘aa’ together”. The former has no basis in Islam; there is 
    no proof that the Prophet (blessings and peace of Allah be upon him) 
    remembered his Lord (i.e., dhikr) with his Companions in unison, or that he 
    would remember his Lord (recite dhikr) and his Companions (may Allah be 
    pleased with them) would repeat after him. 

    But with regard to offering du‘aa’ together, there is a basis 
    for doing so in Islam, and it may take many forms. In Qunoot an-nawaazil 
    (du‘aa’ of qunoot at times of calamity) and qunoot in Witr prayer, the 
    Prophet (blessings and peace of Allah be upon him) would offer du‘aa’ and 
    his Companions would say Ameen behind him. The majority of scholars are of 
    the view that the worshippers should say Ameen to the du‘aa’ of the khateeb 
    on Friday, and when praying for rain (istisqa’), and there are many other 
    various ways of offering du‘aa’ together. 

    With regard to innovated (bid‘ah) ways of offering du‘aa’ 
    together, there are several forms: 

    1.When a Muslim calls together a 
    group of people solely for the purpose of offering du‘aa’

    It was narrated that Abu ‘Uthmaan said: A governor wrote to 
    ‘Umar ibn al-Khattaab, saying: Here there are some people who gather 
    together and offer du‘aa’ for the Muslims and the governor. ‘Umar wrote back 
    to him saying: Come (to me) and bring them with you. So he came, and ‘Umar 
    said to the doorkeeper, Bring me a whip. When they entered upon ‘Umar, he 
    began to strike their governor with the whip. 

    Narrated by Ibn Abi Shaybah in his Musannaf, 13/360. 
    Its isnaad is hasan. 

    2.People gathering to offer 
    du‘aa’ in unison

    Shaykh Bakr Abu Zayd (may Allah have mercy on him) said: 

    Dhikr recited together in unison, in secret or openly, where 
    a specific du‘aa’, narrated in texts or otherwise, is repeated, whether that 
    is done by everyone or one of them prompts the others, with or without 
    raising the hands – all of these are actions that require a shar‘i basis 
    from the Qur’an or Sunnah, because that comes under the heading of worship, 
    and acts of worship are based on tawqeef (i.e., they should be limited only 
    to that which is mentioned in the Qur’an and saheeh Sunnah) and on following 
    (the Qur’an and saheeh Sunnah), not on innovations and inventions. Hence we 
    looked in the texts of the Qur’an and Sunnah, and we did not find any 
    evidence to support this form of dhikr. Hence we can be sure that there is 
    no basis for it in sharee‘ah. Anything for which there is no basis in 
    sharee‘ah is an innovation (bid‘ah). Therefore dhikr and du‘aa’ that are 
    done in communal form are innovations and every Muslim who follows the 
    example of the Messenger of Allah (blessings and peace of Allah be upon him) 
    must refrain from and avoid them, and adhere to what is prescribed. 

    Based on that, offering du‘aa’ together in unison, whether it 
    is du‘aa’ at any time or following a certain activity, such as after reading 
    Qur’an or after an exhortation or lesson – all of that is innovated. 

    Tasheeh ad-Du‘aa’, p. 134, 135 

    With regard to the du‘aa’ of a lecturer or teacher at the end 
    of his lesson, and the audience saying Ameen to his du‘aa’, it seems to us 
    from the Sunnah of the Prophet (blessings and peace of Allah be upon him) 
    that this is permissible, and indeed mustahabb (encouraged). 

    It was narrated that Ibn ‘Umar (may Allah be pleased with 
    him) said: The Messenger of Allah (blessings and peace of Allah be upon him) 
    rarely left a gathering without offering these supplications for his 
    companions: “O Allah, give us a share of fear of You that will prevent us 
    from disobeying You, (a share) of obedience to You that will help us to 
    reach Paradise, and (a share) of certainty that will enable us to withstand 
    the calamities of this world; cause us to enjoy our hearing, sight and 
    strength so long as we are alive, until we die; avenge us against those who 
    wrong us; support us against those who wrong us; do not make our calamity in 
    our religious commitment; do not make this world our main concern and all 
    that we know about; do not send against us those who will show no mercy to 
    us.”

    Narrated by at-Tirmidhi, 3502; classed as hasan by al-Albaani 
    in Saheeh at-Tirmidhi. 

    An-Nawawi included it in his book al-Adhkaar, in a 
    chapter entitled “Du‘aa’ of a person in a gathering for himself and those 
    who are with him.” 

    Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) 
    was asked: 

    Sometimes, after giving a lecture or a lesson, the lecturer 
    offers du‘aa’ and raises his hands; should we sit with him during the 
    communal du‘aa’ or should we leave after the lecture, before the du‘aa’? 

    He replied: 

    There is nothing wrong with offering du‘aa’ after a lecture, 
    exhortation or reminder; there is nothing wrong with offering du‘aa’, 
    calling upon Allah to help and guide those present, and to grant them good 
    intentions and good deeds. But I do not know of any evidence for raising 
    their hands in such cases, and I do not know of any report to that effect 
    from the Prophet (blessings and peace of Allah be upon him) except some 
    texts that speak in general terms of raising the hands when offering du‘aa’, 
    and say that it is one of the means of having the supplication answered. But 
    I do not remember any report from the Prophet (blessings and peace of Allah 
    be upon him) to suggest that after he had exhorted or reminded the people, 
    he would raise his hands and offer du‘aa’. If he used to do that, the 
    Sahaabah (may Allah be pleased with them) would have reported it, because 
    they did not omit anything but they reported it (may Allah be pleased with 
    them). So it is preferable and more on the safe side not to raise the hands 
    in such cases, unless there is evidence to that effect. With regard to the 
    speaker offering du‘aa’ for them after he has finished speaking, and saying. 
    “May Allah forgive us and you” or “May Allah guide us and you,” or “May 
    Allah cause us and you to benefit from what we have heard,” and so on, there 
    is nothing wrong with that, and if they say Ameen, there is nothing wrong 
    with that either. End quote. 

    Fataawa Noor ‘ala ad-Darb, tape 
    no. 610.