Dumfries area is probably your best bet for the most options – Ae and Mabie are about 10 mins, Dalbeattie and Drumlanrig 25 mins, Kirroughtree about 45 mins and Glentress/Innerleithen about an hour and a half away.
I’ve stayed at the pods at Mabie house hotel a couple of times and they are great for a group – they sleep 3 comfortably (4 at a squeeze) and there are 3 of them. They are cheap (£40 per pod), pretty much on the trailhead for Mabie and you can get a cooked breakfast for £7 at the hotel. The food at the hotel is also pretty good.
http://www.mabiehousehotel.co.uk/pod-continental-camping-huts-c23.html
Also, checkout Mathrown of Mabie campsite as they have bunkhouses available too and is just along from Mabie house hotel.
http://www.marthrownofmabie.com/