This is determined during booking, ideally all assessments would be taken in a Private format, 1-to-1 with an instructor. We do allow experienced riders (years of experience, own horse etc) to take an Assessment Lesson in a Group Lesson, providing they feel confident enough.
You must be able to demonstrate proficient Walk, Trot and Canter, as an example, the first 2 motions, and probably the third  with no stirrups whilst doing exercises. If you can't do this, or think you might fall off, then a Group Assessment isn't suitable for you.