Hi my brother and I are new to the server and want to make / join a coop but we don't know how can anyone help us? Thank you!

 

Make whoever's island you want to keep type /coop invite <your name>. So, if your brother wanted to make you join his island, he'd type /coop invite Abbie (or whatever your in-game name is).

Does this answer your question?

 

Archiving from inactivity, I'm assuming that the question was answered. Feel free to open a new thread if you have further questions.