Bansko
The best all round base for a large group, with the biggest ski area, the most beds and the loudest nightlife.
Bansko is the obvious first choice for numbers. It has the largest and most modern ski area in Bulgaria, served by a gondola straight from the edge of town, plus by far the deepest pool of hotels and big apartments, so even a group of fifteen or twenty can stay together cheaply. The old town's mehana taverns and the strip of bars and clubs give a crowd plenty of room to spread out at night.
Sofia is around two and a half to three hours by coach, the longest of the mainstream transfers but easily booked as one group vehicle. Choose Bansko when the priority is keeping a big group together with maximum value, decent skiing and the best nightlife in the country. It is lively, so peace seekers should look at Pamporovo.