In reply to nodwas:
You don't have to be an MBA member but they ask that groups of more than 6 avoid bothies. Glen Feshie is smaller than it used to be now that the upper floor is closed off. Feshie is also very popular over weekends and theres a fair chance that other groups will be trying to use it too so even a group of 10 would be too much imo.
Bear in mind that if you do decide to use a bothy for a week, there will be other users looking to stay as well most of the time and room would have to be made for them if they turn up (i.e you can't turn folk away because you're using it).
A good relatively cheap option are club huts - Have a look at:
http://www.mcofs.org.uk/find-a-hut.asp
Normally a hut will be in the region of £6-10 per night per person and you'll get the place to yourselve in most cases.
Failing that you'll most likely be able to book 10 places in a hostel rather than booking the whole hostel.
Ardenbeg bunkhouse in Grantown on Spey or maybe the Carrbridge Bunkhouse would be good options as well.
Mill cottage by Feshie Bridge is a nice hut to stay in.
I can heartily recommed the smiddy by dundonnell for a great hut too.
Regards,
Stuart