The user who creates the Team becomes the Team administrator. An administrator can manage the Team account: invite and delete team members, allow them to add new Team members, change billing information, renew the subscription or delete the team. A Team can have several administrators, as the Team administrator can provide Team members with administrator rights.
A Team consists of a certain number of paid seats. When members are added to the Team, they start using the paid seats and get access to all the features available in the Team plan. Team members can be invited and deleted, so different people can use the paid seats while the Team is valid.
Board Participants (External collaborators)
Board participants are those users who are invited to specific boards with Viewer or Commenter rights. They don't necessarily need to be a part of your Team account to be invited to your boards. Any RealtimeBoard user can be invited.