In order to create a track, you'll need to go to the Plan page. Click on the "Manage" button in the box directly to the left of the Plan calendar.
You'll be able to add up to 4 tracks at your gym. Click on the name to change your label for it then decide whether you want it Open or Closed. An Open track allows any of your gym's members to join it. A Closed track keeps the group private. You'll have to add the members yourself via the "manage members" link.
Add members to a given track by clicking the "add" button next to the athletes name. There is no cap on how many people can be in a track. Remove members from the tracking by clicking the "remove" button