GIVE ANSWERS - English

The subject of infinitives in English is vast and complex. They have so many uses that listing and describing each is rather a difficult task.

In the question Are you going to the party held in their office?, Are you going to the party is the independent clause and can act as an interrogative sentence on its own. If you really think about it, it is true. Are you going to the party? is a perfectly valid question. In the case both participants know exactly what party is being referred to, there's no ambiguity. The problem arises when the person you're talking to is attending multiple parties or forgot about this particular one. In which case, it is only natural for them to ask What party? to determine context, to which you reply with: The party (that's going to be) held in their office. It's worth mentioning that held (past participle of the verb hold) is not an infinitive here. It's modifying the noun phrase the party to distinguish it from perhaps another party that Mike's holding at his place.

You can use an infinitive with Are you going to the party to ask for a reason or purpose, e.g. Are you going to the party to have fun? (of course you're going there to have fun). to have fun is acting like an adverb and modifying the whole sentence.

Short summary:

Are you going to the party held in their office? — what party are you talking about?

Are you going to the party to get drunk? — why are you going there?