Many years ago I did a 'safari' with Abercrombie & Kent to the Serengeti. The group, (mostly not known to each other), comprised folk with very different ages and backgrounds - all we had in common was the ability to pay for an up-market trip.
One section of our party was a group of anaesthetists travelling together. As an aside, they were an absolute pain in the arse - a bunch of little Englanders who treated the local staff like rubbish.
Moving on, it occurs to me that your target market might be groups of relatively affluent professionals so it could be worth getting some high-quality leaflets printed and sticking them up in places like hospitals, golf clubs, tennis clubs etc inviting people to form their own group and offering group rates.
Just a thought.