Here's what you need to do;
1. Login to clubs.teamstuff.com
2. Jump to the People tab, and search for the player you want to change.
3. Now click on them to see their full details. It will look something like the following;
4. In the Team: section you'll see a listing of all their current teams. If you want to take them out of one of those teams - just click on the Remove. Note : there's no pesky 'confirm box' here - so make sure you really want to remove them from the team.
5. If you wanted to add the player to a team - try the green Add to a team box.
When adding people to teams, you can add them to any role - so you can use this for reassiging coaches and managers just as easily as moving players between teams.