An initial assessment is essential regardless of the class size. I always meet with my first time bootcampers and put them through a short bootcamp workout, measure them, and review their forms, this takes approximately 30 minutes of my time but it is very important to me.
As for personal training clients, i spend 90 minutes initially doing a full assessment.
For group exercise classes I would always talk to new people prior to class and would show them modifications if need be.
As always, release forms, PARQ forms, need to be filled out. If you cover your concerns with these forms you will have information at your fingertips.