People who are in the Web App Group i.e. Web App Users are allowed to access the Web App and count towards your seats quota. Pending invitations for users to be added to your Web App Group also count towards your seats quota.
People who are in the Admin Group can access the Organisation section of the app.
They can manage things like:
People who are in the Admin Group can also access the developers section if your Organisation has an API plan.
A person can be in either one or both groups.
To check which team members are in your Organisation's Web App Group: