You can invite as many staff as you want to your account. Only users with the Administrator role can invite new users. Click the gear on the top menu and select Users. Click "Invite a New User". Fill out their name and email address and optionally time zone. After you click "Create User", their user account will be created and they will be sent an email invitation to Advancement Form.
By default, new users have no permissions. After the invitation is sent, you are directed to assign the permissions for the newly invited user. Consult Roles and Permissions for more information on the roles and their permissions. After you have assigned their roles and clicked Update User Permissions, their user account is complete.