In Certain Arrive, a user is called a Team Member. To add a new team member, click on your name at the top right corner, and select Team from the dropdown. Here you can click +Add a New Team Member. You are able to add their name, email and phone number, but also select whether they are an Administrator or a Planner.
The Planner can add/edit/delete events and add/edit/delete/check in guests. The Planner does not have access to billing related functions and cannot add or delete a team member.