Denrus has a calendar board that you can post yourself to if you are looking to share a tour. I also posted on CruiseCritic.com "roll call" faor our sailing date.
I got over 30 responses and have now put together a nice private tour with 8 of us who seem the most compatible in our desires. we're on the 5/25 sailing.
you can call Denrus. they have a US telephone number on their website. denrus.ru
they are an american company with full insurance and accept credit cards which tipped the scales for me in choosing them, particularly after the Chile crash disaster. they have early entrace to major sites, you go right to the front of the line. they have many ears of experience meeting cruise ships. they are the current official shore excursion provider for P & O out of Britain.
i'll post here when we get back 6/5 to let you know how things went for us.