A coach or player will need to register for each Team they wish to be rostered onto. So, the coach or player will either need to register on their end for each team they wish to be on.
Or, if you would like to register the coach or player as an Admin, follow the steps in this article:
https://teamsideline.zendesk.com/hc/en-us/articles/201153157-As-an-admin-how-do-I-register-or-place-an-order-for-my-customers-.