、？
What time should I gather in the lobby tomorrow morning?
Literal
Tomorrow [possessive-の] morning, what time [at-に] lobby [to-に] gather [if-ば] good [polite-です] [question-か]?
The pattern ~ばいい means 'it would be good if [I/we] X' → 'should X' when used in questions. Literally '[gather], if [I do that], is it good?' — the Japanese way of asking 'what would be the right thing to do?' 集合 is the Sino-Japanese noun 'gathering/assembly,' verbified with する (here in conditional form).