How sound is the hadeeth “Husayn is of me and I am of Husayn”?
Question

I passed by a booth in my university that looked Islamic. I've learned to pay close attention to the writings on the posters and the books, for I once saw what looked like a well-presented Islamic booth but it turned out to be Ahmediyyah, because of that huge portrait of Mirza. Anyway, I came across one called "Thaqalayn". I've never heard of that group. Who are they? Also, they had posted the following: "Hussein is from me and I am from Hussein" (narrated by numerous sources including Imam Ahmad ibn Hanbal, Al Tabarani, & Mishkat al Masabih). Is this an authentic hadith?.
Praise be to Allah.The hadeeth mentioned was narrated by al-Tirmidhi (3775), Ibn Maajah (144) and Ahmad (17111) from Ya‘la ibn Murrah (may Allah be pleased with him) who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Husayn is part of me and I am part of him. May Allaah love those who love Husayn. Husayn is one of my grandsons.” 
This hadeeth was classed as hasan by at-Tirmidhi and al-Albaani. 
This hadeeth points to the virtues of al-Husayn (may Allah be pleased with him). Ahl as-Sunnah love al-Husayn and respect him; they take him as one of their own and they testify that he is in Paradise, but they do not go to extremes in that as the Raafidis and Shi‘ah do. So they do not call upon him instead of Allah, and they do not believe that he is infallible or that he has knowledge of the unseen, and they do not hate the Companions of the Prophet (blessings and peace of Allah be upon him) and they do not regard anyone of them as a kaafir; they do not slander Abu Bakr, ‘Umar, ‘Aa’ishah or anyone else among the Sahaabah. 
There is the fear that the Thaqalayn group may be a Shi‘i group, so beware of that and adhere to the path of Ahl as-Sunnah and the mainstream of the Muslims. 
We ask Allah to help and guide us and you. 
And Allah knows best.