By default when you first install Simple:Press, all new users will be placed in the ‘Members’ User Group. As you create your own User Groups you can also map WordPress Roles to Simple:Press User Groups.
WordPress uses a system known as ‘hooks’ which is the method by which WordPress and plugins communicate. Think of it as a sort of messaging system.
When a new user registers on WordPress, the process sends out a ‘message’ so that any plugins listening for that message can act accordingly. This is how Simple:Press discovers a new user and creates the necessary data for them to access your forums. This process includes User Group membership.
The two most common reasons for this process to fail have been:
The simplest way to test for this is to manually create a new user in the WordPress New User admin panel and then see if they were correctly allocated into the designated Simple:Press User Group. Assuming that the second condition above is not a factor, if the allocation happened correctly then the fault is most probably with the alternative registration system and help should be sought from their support.