[Mivereous]

I just figured it would prevent most bugs/grief issues.

I.e.,

Player A + B vs Player C
Player A runs up to Player B and uses Explosion; Player A switches teams right then; who would Explosion affect?

Player A, B, C, D vs Player 1, 2, 3
Player 1 switches teams; suddenly, 5v2. I know this one is unlikely, but we've had obvious dumbbugs before.

Player A + B vs Player C
Player A just switches teams every time Player C is targeting. Immune to all damage.

Player A + B vs Player C
Player A is faster than Player C, and runs up to Player C and switches teams and skips turn; Player C cannot harm them, and thinks they have a new ally; Player A's turn rolls back around, and Player A now switches back to the team with Player B, and has one and a half turns to use against Player C.

Even if you added a pop-up that asks the other team's leader if they wanted to allow you in, there's still that stupid thing where Spacebar triggers your prompt answer, which would be just as annoying as when someone invites you to a team/challenges you while you're typing.

Making it a thing that can just happen on the drop of a dime without some kind of repercussions is just a bad idea, I would think.