There are 5 of us planning to come. How much time should we allow from arrival to completion?

As busy as I saw it getting, you could be waiting 20 min at front desk to get everyone checked in. Your coach will be waiting for you and fetch you from the lobby. Still probably worth it to show up 45 min ahead of scheduled time.

If you do a morning session and after lunch... then 5-6 hours.

It depends on what type of sessions you're doing. On their website you can book for any of the sessions but if you want to do a group, you might want to call as they have only so many cars and session types available. I reserved ahead of time and walked on about 15 minutes early and went out to the track within 5-10 minutes.

If your planning on driving, this needs to be booked in advance, they rarely have any day of openings. Cool place to swing through as mentioned for lunch too.

That's a tough question to answer without knowing what you want to do at the facility. You could just walk in and see the cars in the lobby, or you could come in for lunch, or you could arrive and drive several cars.