When a permission is set to Team only, users can only access records owned by or assigned to users in the team for that specific permissions.
When a user requires access to another team's records or content, an Admin with the Add and edit users permission, or a Super Admin, can assign a user as an extra team member. They'll then be able to access the additional team's records and content with Team only permissions. Extra team members can also access main team members' owned contacts.
When a user is added as an extra team member:
the user can access the team's records and any content that's set to Team only permissions.
the user can access main team members' owned records.
the main team members can access the extra team member's records.