On my trip to Tuscany in May there was a group of 12 people who'd met on a holiday the previous year booked on our holiday. Some of them were ok and mixed, but others wouldn't and since they had their own tables for meals we just kept out of their way most of the time.
I did complain to Just You about it but they replied that they didn't know they'd booked it as a group - which is fair enough.
Other than that I've encountered a few sisters, friends, and even a couple on my last holiday, but never more than 2 people so it's never really been much of a problem, and they've always mixed with the rest of the group. The group of 12 may have been a bigger issue had the group been small though.
I notice in the small print at the back of the Just You brochure it states, under 'Group Reductions' - 'we are able to offer reductions for groups of 10 or more people on most of the holidays in our brochures' - it then quotes a telephone number to ring to enquire about group reductions. Can I just ask why this is stated in a Just You brochure? Really a company who specialises in holidays for single travellers (as stated on the front cover of the brochure) shouldn't be encouraging group bookings?