Like they can invite new users, administrators of your account (i.e. users with the administrator role) can grant or remove any other user additional role. 

Here is how to modify roles for an existing user:

  1. Log in to your Myrmex account on https://login.myrmex.coop
  2. Click on the "Users" Tab in the left menu :

  3. Click on the "Edit roles" button : 


  4. Pick the user, then check the boxes next to the appropriate role(s) for that user (you can't remove the Viewer role, and should inactivate the user account instead) :


  5. Click on "Save"

If you want to know more about roles, please refer to this solution.