To add someone else as an admin, they first need to register as a regular user.
Once they've signed up, follow these steps:
Click on your avatar in the top right of any page, and a menu should appear.
Click "Site Admin".
Underneath "Users" on the left, click on "Overview". You should now see a table of the members who are signed up to your site.
To the left of each name should be a small tick box. Tick the tick box next to the user you want to make into an admin, then click on the dropdown at the bottom of the table that says "Bulk actions..." and choose "Promote to Admin".