Saul, An 'it would be great suggest' is not a check with CF staff member about 'Why there is no 'join button currently on both sides of a team match'; 86 views, I only one to respond, with anything.
"pick a side" button for team matches

this is only a suggest for site, i think will be good if members can join at one side even when they are members in both groups,more members could be joined at matches for play without having to quit and rejoin at groups.
just like in vote chess that you have the option of pick a side to play for
Why don't you make this 'suggest' where someone other then me might comment on it; say like in Cheating Forum Team Notes area?

i made here cause i think there is a moderator of chess staff and he/she will see it .
is good idea to post at that group where Eric ( the owner ) is member too

This isn't a bad idea, but I think there might be some concern that a player might, say, join the opposing team in order to play against their real team, and throw the game to help the opposing team. Of course, I doubt most people would be so unsportmanlike, but it might be best just to not allow a conflict of interest.

good point ArchdukeShrimp .

So let's say in a vote chess game, I'm in two groups. Winners group (real) and losers group (possibly real) . I join the vote chess as losers group and throw the game. If that was a team match, the results would be bad if only two people played in the team match.

pssst! why u did not mention my name enrrikerrr? lol! what is pick a side?
i think could be great if site install a button at team matches that let you join to a group when you are member in both groups of the match (similar to vote chess option that let you pick your side to play )