Changing Teams in Civ 5

Solution 1:

Being on the same team as another player achieves the following:

• Permanent ally throughout the game. There is no way to break your alliance.
• You are unable to attack the opposing player.
• You share technology and research points.
• Shared vision
• If one of you wins, you both win.
• Shared diplomacy with other civilizations and city states (i.e. if one of you is friendly with a civilization you both are).

One additional thing I'd like to note that is if you are playing in an online multiplayer game and one of the players is wiped out, they are kicked out of the game whether or not any of their allies are still alive.