No. There are no additional fees for adding members to your shared team number. All you need is one active subscription that you have activated to a Team Number account.
Additionally, any members that you wish to add to your shared number do not have to have their own Sideline subscription to be added to your team. In fact, the invite process is easiest if they do not have an existing Sideline subscription.
For instructions on how to send/receive a team number invite, please visit 'How to send & accept a Team Number Invitation'
Questions? submit a request.