Aside from places like here, maybe could try meetup.com
There are loads of MTB groups and trips arranged on there, though there’s a pay element to it, usually by the person who arranges it, but perhaps someone’s got something planned that may fit the bill.
Edit: In fact just noticed there’s one in the group I’m a member of – http://www.meetup.com/London-Mountain-Biking-Club/events/189648282/
(you’ll probably have to sign up to view it, but should be free to sign up)
Maybe not quite what you’re after though. A bike & backpack tour going from place to place it seems. Still, you can also suggest meet ups for free I think.
There are also the various MTB holiday companies based in resorts (a fair number in Morzine for example). Though they mainly deal with groups booking a chalet and not the transport, it might be worth contacting some and see if any of their bookings are looking for extras to bump the numbers, etc.
If you’re okay with tagging along with people you don’t know that is.