Before a user is able to sign in, they must be added to the Flywheel site by a Site Admin.
The Site Admin does this by clicking on the sidebar Admin -> Users option and then clicking on Create New User. After entering the users name and email address, the admin is able to go to the Permission tab to add permissions for that user to join a Group and Project. The admin can also give that user admin, read/write, or read only permissions for each.
A demo is provided below.