Not all races offer the option of signing up a team, but if they do, you'll need to select the 'TEAM ENTRY' option on the event page.
Once on the entry form, you'll need to click 'Or create a team', select a preferred wave (if applicable), your team name, and other team preferences.
If you want the team to be private you can add a password (see additional notes on private teams here). You may also select you're estimated team size, which may come with a team discount (see more on team discounts here).
After you've completed your entry, you'll be taken to your Team Page, where you can invite others to join.