When inviting a new Account user, you can choose to check the "Invite as administrator" box. The user then becomes an admin when he/she accepts the invitation.
If you wish to assign a user admin permission that has already accepted an invitation, navigate to the "User" menu and click on the "admin" button.
Inviting a Segment user admin, you first invite a Segment user, they accept the invite. Then go to the Segment user list and click to make admin in the list on the person you want to give Segment user admin access.