If you have a premade team, I think KI expects you don't need team up. Were you trying to get a fourth player to fill the team? When it first was introduced, I don't think you could use team up if you were in a group.
If you have a pre-made team, then enter at the same time. There was time when there was a glitch that occurred if you were grouped ( I don't know if this is still going on. )