In Inspirica Groups, a "seat" refers to a spot in the class that can be filled by a student. When all the "seats" are taken, no more students can be added to the group, and therefore no more students can enroll in the course as part of this group.
If you are a Group Leader that would like to add more students to a full course, you can add more seats to your group from the group management page.
When a student is invited to a course via email, they occupy a seat in the group. If a student is instead sent an enrollment key, a seat is not occupied by that student until the enrollment key is redeemed.