Sometimes it takes a team! If you'd like to give admin access to your site to a collaborator, invite them via their email through the Users account tab.
The invited member will receive an email with your logo, and button to claim their invitation. An e.g. email is used below for our CG Cookie project.
Clicking the Claim Invitation button will bring the invitee to a Create an Account page. Once their account is created they will be added as a collaborator to your site, and listed as a user on the invite page.
An Unclaimed status will be show if the invited user has not claimed their invite. Clicking the link icon will also enable you to copy/paste the invite link directly in a chat communication or email with the invitee.