How to join a nation

As a town mayor you can join a public nation with command /n join [nation]. Keep in mind that your town has to be within 3500 block range of nation capital.

If the nation is set to private, ask a nation king to invite your town to the nation. The king has to run the command /n add [town] and the town mayor has to accept the invitation with /confirm. Once that's done, the town will be added to the nation.

Check out the nation documentation page for more information.