With a coach membership you are able to setup and/or maintain unlimited athlete profiles. Under the "My Athletes" tab you can add athlete profiles yourself or invite athletes to setup their own Best Bike Split memberships and then have them grant you access to their account.
Other than having the option to work with multiple athlete profiles, a coach membership essentially functions just like the premium athlete memberships. Once you select the athlete profile that you wish to work with, you have the ability to add/modify that athlete's profile, add/modify that athlete's bike(s), add/upload courses for that athlete and create/modify race plans.
The coach membership does have an additional link on the race details page called "Athlete Link" which is a link that you can send your client/athlete giving them access to their race detail page even without being a Best Bike Split member.