1. Open the main menu (three horizontal lines on the top left)

2. Click on "Organization"

3. As an administrator, you will see under the heading "Members of my organization" your members and the requests (descending from newest to oldest date)

4. You can click on the user for whom you want to make an administrator

5. Now the option "Make Administrator" appears here

 

Please note, when you make a user an administrator, you cannot delete them.