I suggest add 2 new buttons on team window

1: Join
If a single player wanna recruit another player but that player is already in group, the "Join" button appears for request to join the team of that player, and then the team leader will receive the join request to choose if allows the single player to join his team.

2: Combine
If a group leader wanna recruit another player but that player is already in group, the "Combine" button appears for request to combine those 2 team, and then another team leader will receive the combine request to decide. If the total number of mates will be more than after combine 2 teams, just remind the group team it's not possible because of extra mates.
I suppose it's better than split one team to let 2 teams combine...