There may be other ways to do it, but like the Friend Invite guide I will explain the quickest and easiest way for now.
The easiest way to do it is to meet your potential new team-mates in-game, bring up the ESC menu and click on the menu item that’s circled in red in the above screenshot. It will bring up a menu on its right. Click the top item in that.
When you click it, this window will pop up asking if you’re sure you want to send this person an invite. The name displayed will be the target’s Player ID, not their character name. Click on the left button to confirm and send them the invite. Your work is done.
I don’t have any screenshots of how to accept a team invite, as soon as I get some I’ll expand on how to accept them.