Brethren (adelphōn). Codex Bezae has “disciples.”
Multitude of persons (ochlos onomatōn). Literally, multitude of names. This Hebraistic use of onoma = person occurs in the lxx (Numbers 1:2; 18:20; 3:40, 43; 26:53) and in Rev 3:4; Rev 11:13.
Together (epi to auto). The word “gathered” is not in the Greek here, but it does occur in Mat 22:34 and that is undoubtedly the idea in Luk 17:35 as in Act 2:1, Act 2:44, Act 2:47; 1Co 11:20; 1Co 14:23. So also here. They were in the same place (to auto).
About a hundred and twenty (hōs hekaton eikosi). A crowd for “the upper room.” No special significance in the number 120, just the number there.