Dribl supports the ability for players, coaches and managers to be mapped or assigned to multiple teams.
By default, the Team Mapper tool only displays members that have not yet been allocated to a team in the Available Members list.
If a player, coach or manager is already mapped to another team, click on the Allocated 
toggle button on the Team Mapper to display these members.
If they still don't appear as an available member, chances are that there are Allocation Restrictions being enforced on the competition or individual member.