What version of Sins are you playing?  Most of the action is on the Diplomacy expansion during the North American evening and on the weekends.  The player counts are low, but you can still find 4v4s and 5v5s.

Your best bet for finding other people to play with is to play on the Diplomacy expansion.  The player counts for this game are low, but you can still find 4v4s and 5v5s from around 4 pm to Midnight U.S. Eastern Time with most of the action being around the European evening (say 4-7 pm U.S. EST).

Note that most of the people are either pros or have large amounts of experience, so plan on taking some beatings (nooby lumps) while you learn the online game.  It's much more challenging to play against other people because they don't do silly things like the AI does.