If you are sharing IP with other players and need special permission, read the rules (to avoid get banned) https://l2reborn.com/info-rules/
For more then tree accounts get in touch with the staff to get a special permission:
Create a support ticket with the topic “Licensed Player” specifying all the accounts (usernames) that need to be authorised.
Once you’ve been authorised you’ll be able to play together, but you need to login one by one, waiting one minute after the previous account has logged in.
Note: Dualboxing is NOT allowed in our server and accounts requesting this special permission will have to pass strict security checks, those that fail it will get permanently banned.