joojoo, the ship will have a bulletin board in the guest relations area. You might try posting a note there asking if anyone is interested in forming a small group.
Another way is to simply get off the ship and go over to where the independent taxis are. Other like minded people will be headed over there also, so you can work up a group with them.This last approach, however, precludes you from booking a ship tour!
Forming an extemporaeous group like this works well in the Caribbeann, but did not work for us in Europe.
We just got back from Europe two weeks ago, but only put into Lisbon, and four ports in Spain. None of those ports had taxis or tour operators with minibuses or SUVs. The only taxis were the standard sedan types that would only sit four people - three in back and one in front.
I don't know if the same holds true for other European ports.If it does, then forming a larger group will not benefit you.
What we wound up doing in Portugal and Spain was taking city tours on those double decker-open top tour buses. They cost (depending on the city) between 9 and 12 euros each and the fare is good for 24 hours. You can hop on one bus and hop off to sight see and then hop on the next one that comes along. For us this worked well because it was our first time in any of those ports and we wanted to see the city.
As an aside, we tried in several ports to see what a taxi tour would cost and were spectaculary unsuccessful in negotiating anything lower than their "fixed" rate - which was thirty euros per person. This was actually more than the ship was charging for a city tour.
If the double decker bus idea appeals to you, you might want to check the net to see if the cities you are visiting have the double decker tour buses.
One other note - in big cities like Lisbon, one tour route cannot cover the whole city so they have a choice of tours. You can buy a ticket for one tour route or you can buy (for a higher price, of course) a combination ticket that covers several different tour routes.
The bus approach, obviously, works well if all you want is a city tour, but if you want to see something outside of the port you will probably have to hire a taxi or take the ship's tour.
Anyway, that was our experience. Perhaps someone who has visited the other ports will have more information.