。
Let's gather by tomorrow evening.
Literal
Tomorrow [possessive-の] evening [until-まで] [dative-に] let's-gather.
までに marks a deadline — 'by X' as opposed to まで which would mean 'until X.' The distinction matters: まで is a continuous duration up to a point, までに is a point-or-before deadline. 集まろう is the volitional form of 集まる (intransitive, 'to gather/come together'), contrasted with 集める (transitive, 'to collect').