there is a team match between two groups I'm both in. Can I choose which group I want to play in? (chess.com preselected a team for me. I went to the site of the other team = the team I want to play in, but if I choose the team game, I can only join the preselected team)

Follow the rules.

read the poetry!

Can you be more specific, echecs?

You may need to temporarily QUIT the group you don't want to play for, join the other team, then rejoin the first group.

Thanks Artfizz, that may be the solution. I just hope there is a less cumbersome way...