First, follow the link your trainer has sent you, or find a group on the "Groups" page. From the group's home page, click "Join Group" on the left side of the screen.
While some groups are free to join, for others you must pay a fee. If you must pay for the group that you want to join, enter your payment information at Checkout and complete your purchase.
After you click "Join Group" and, if needed, complete the purchase, you'll be added to the group.