An administrator has full access to everything. They can invite other admins, and edit their privilege levels. Create, delete and edit stores, employees, roles, and most importantly quizzes. As well as set the company logo and colors.
A read-only only has access to view things, they have no ability to create edit or delete anything. This should mainly be used in order allow other users to simply view reports.
If an invite goes unclaimed for 7 days the invite will expire, you will have to send them another invite if they wish to join in the future