Administrator role provides much more rights than User role. As administrator, you can give admin access to other users. To do this, open the account drop down menu and go to [Team name] Settings.
You need the tab "Users". Click on the user name you need, and you'll see user details panel on the right. Please press link button "Make administrator" on it.
Metatask will always double-check with you about giving admin access to a user. You can learn more about Administrator role permissions in article User and Administrator roles
You can revoke admin access at any time - press link button "Make user" on user details panel and confirm you'd like to revoke her access.