Hi,
Could you add to your plugin possibility to create separate group for each server (with MySQL) and separate user ranks for each server?
Currently, in database, in table with groups there is only group's name, there isn't a column for server name.
Thanks

In a bungeecord network, you could have a permission that applies on both a server AND a world using context.

Only thing I could say is read the wiki, advanced setup.

Here, I'll do half the work for you.
Advanced Setup
What is Context

Groups themselves are not per-server, they're just assigned to players on specific servers, or given permissions on specific servers.