When used for places it means "between" and goes after the two places it links.
When used with poeple is means "between" or "among" and goes after the people it links.
When used to talk about activities, the subjects of the two clauses must be different.
The conjugation for this one is a little tricky so be careful. If it is an action verb you use 는 사이에 if it is a directional word you'll use ㄴ 사이에 for past and 는 사이에 for present and future.