When you set up the LearnDash Uncanny Groups plugin, you will be given the option to upgrade on the settings page.
When an administrator clicks on the Start Process button, all LearnDash groups created through the standard LearnDash groups interface (including all groups created before Uncanny LearnDash Groups was first installed) will be made available to their respective Group Leaders to manage on the front end. This upgrade will also include establishing the number of seats for each group that is a calculation of Group Leader(s) plus the number of users already in the group. So, everyone in the group will be set as an enrolled user in the group and the available seats will be set to 0.
What if you don’t upgrade groups? Non-upgraded groups won’t be manageable in the front end. They will, however, be available in the front-end reports.