how do i join a group with my friend?

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I'm assuming you mean how to join a "Team".

To join a team you need to get an invite from a already team-member.

To invite you the member then needs to select "friends" (shaped like a heart) in the "social" tab and then left-click on you (or whichever he/she wants to invite) and then choose "Invite to team" in the small list that follows the left-click.

I hope this helped