Put friends in same team in GameLift/FlexMatch when game is created

Yes you can prefill a team and let FlexMatch handle finding matching players for another team(s).

If you know everyone in the match then you can just skip flexmatch and handle the placement yourself (ie StartGameSessionPlacement).

See https://forums.awsgametech.com/t/how-to-implement-a-party-matchmaking/6732/2 for lots of information and links to other useful threads.

Can i just prefill a part of team? For example, in a 5-vs-5 game like Dota, I create a party with just one friend, can FlexMatch support this situation? Thanks a lot.