Navigate to Administrator -> Users
You can either search for a user or filter according to you needs.
When in the user profile, click Edit.
Further down in the new window you will find the Access Group, click on Choose.
Choose the admin-role that you want to assign to the user (note that the list may look different on you portal). Click on OK when you are done.
The new role will be added with the previous one as show in the image below, click on Save to accept the new changes.