I tried to add my friend to a group with /lp user Abu-Ha group add "group name" and i got "invalid nickname/uuid" error, i tried samething using console and i got same error again. I used the /lp editor to add my friend to group, my other friend and my nickname doesn't have a - symbol and i didn't got a error when i am adding myself and my other friend to a group.

The LP config has an option to allow/accept invalid usernames - try enabling that