I just tried to add someone to an existing team, and the /addteam command said it succeeded in the chat, but when I go verify that it worked with the global.teaminfo console command it shows that it didn't work: the player I tried to add is still not in a team.
Additionally, on their end they see they were added to the team, because they see the team UI and can read messages from others in team chat, but they cannot reply back, their messages are not going through.
The team leader was offline at the time I tried doing this, if that makes any difference.