Hello,
What you need to do is double-click on the person be it by clicking on their character or double-clicking their name in the chat. Their information should pop up (biography, username, etc.) where you will then find an option that reads "Invite to Group." It should be located underneath the "Send Friend Request" option but I could be mistaken via precise location as I am not in-game at the moment to check for myself.
Arr, Hakumi beat Vespian to the punch.