It is possible to join a server through direct IP.
My friend is playing Terraria and wants me to join him, but we are in different places.
To join him/her you need to go to settings, find VPN, and as a VPN server IP enter his/her router's IP.
Then you should be able to see his/her game just like if you were playing on same WI-FI.
I've never worked with LuckPerms, but what I understand from the default groups wiki page on github, is that luckperms automatically assigns the group default to a user (correct me if I'm wrong.) However, there is a way to take away this group, and replace it with another in your luckperms.conf.