You can only add persons who are members of the current account when browsing Consortium groups because they are specific to the current account. In order to create 1..n groups and 1..n people to add to those groups, it is best to log in as the account owner. You won't obtain the desired outcomes if you try to perform this using the primary administrative account.
You are shown with all of the users associated with the current account while loading the group interface.
Group Interface
The interface displays add-a-group controls when adding a group. The user selection control only displays options for users who are currently logged in to the account.
To avoid losing access to the menu choice for the admin dashboard, do not add your primary admin user to a group. Instead, test this with a different user.
Add Group