This is relatively self explanatory, but it does take some time to find *5 willing gamers to send invites to their team.
*As noted in the comments by planting42, One person can be used. They just boot you and re-invite you. Thanks for the update, planting42.
Simply click on a player in the world, hit the chat icon located at the top of the H.U.D (it looks like a bubble with some dots inside it) and simply type a request for an invitation to join them on their team. It will send a private text to that gamer.
You could also pull up the world chat option by hitting the red arrow found at the bottom of your screen. Hit the chat icon located in the top left corner of the chat box (it looks like a bubble with some dots inside it) and simply type a request for an invitation to join a team.
examples: Invite to team, please.
Team-up invite, please.
Looking to team up.
Most gamers are willing to quickly invite you to a team. It is then your choice to play or drop the team. Do this with 5 different gamers. Pop goes the achievement!
To drop the team, select your character's main menu, located at the top left corner. Then, choose the team tab and select the icon that has an arrow pointing through a door.
*Inviting people to your team will not net you the achievement.