AutoTeam

This mod will automatically assign all players to a team after one person dies. However, it will not do that until the command "/setupteam <displayName>" is run. This command creates the "players" team with the given display name. There is a delay though to the Auto-Assign due to Corail Tombstone's Ghostly Effect apparently causing the command to not run right as the player respawns. Currently, the delay is set to 100 ticks (about 5 seconds) after a player respawns and joins the world. 

I just want to say, if you don't want everyone to be on the same team, don't install this mod. Simple.

That is quite literally all this mod does.

I pretty much made this mod so that I didn't have to constantly do "/team join [team]" every time I died when playing with friends.