Is Tarawih Compulsory?
Question

What is the ruling on a person who does not pray Tarawih at all during the month of Ramadan with no valid excuse? Is there any sin on him because of that?

I work for the Aramco Company and sometimes my work requires me to stay until after Maghrib, so I have to break my fast at work. I am just about the only Sunni Muslim there, the rest are Shi’ah and Isma’ilis. Is it permissible for me to break my fast with them?
Praise be to Allah.Is tarawih compulsory?
If a Muslim does not pray Tarawih , there is no sin on him for that, whether he has an excuse or not, because it is not obligatory. Rather it is a confirmed Sunnah which the Prophet (peace and blessings of Allah be upon him) did regularly and encouraged the Muslims to do as he said: “Whoever prays qiyam in Ramadan out of faith and in the hope of reward, his previous sins will be forgiven.” (Narrated by al-Bukhari, 37; Muslim, 760) 
But the Muslim should not neglect Tarawih prayer . If he cannot pray it with the imam in the mosque, then he should pray it at home. If he cannot pray eleven rak’ahs then he should pray as much as he can, even if it is only two rak’ahs, then he should pray Witr. 
Is breaking fast with the Shi’ah and Isma’ilis permissible?
With regard to breaking your fast with the Shi’ah and Isma’ilis, if you think that breaking your fast with them will serve the purpose of softening their hearts and calling them to follow the Sunnah and give up the innovations that they are following, then this is something that is prescribed in Islam.  
But if you think that your breaking your fast with them will not serve any purpose, then it is better not to break your fast with them, and to avoid them, so as to denounce the innovations that they are following and lest they confuse you with their specious arguments when you do not have enough knowledge to realize that what they are saying is false, thus exposing yourself to fitnah (temptation, tribulation). 
And Allah knows best.