To add a coach to a team you need to configure the Team Coach field in your Settings. To do this:
1. Click the Cog on the right hand side of your dashboard and then click Settings.
2. Click Field Configuration.
3. Click Team Fields.
Now this field will display when you are editing a team.
For a member to appear in the drop down list, they need to be registered in the current season as a coach. This can be configured at the member level (member history > clubs > edit clubs).