Our group of 4 had different experiences with this on our trip to/from Hawaii.
On the way down only one of the group had the pre-check. The TSA officer told all 4 of us to enter the pre-check line together, but when we got up to the belts, the three of us who didn't have pre-check had to split off onto a separate belt they had set up beside the pre-check one.
On the way back, 3 people in the part received pre-check, but I'm assuming since my last name was different, I wasn't selected. I told one of the TSA officers my husband was selected and I wasn't, and they stamped my pass that I could go thru the line as well.
I really think it depends on which airport you're at, but it doesn't hurt to ask.