Talk to a local travel agent. Find one that specializes in European vacations and tell the travel agent where you wish to go, what your focus is, and your preferred method of travel. Examples may be touring French vineyards with other retirees traveling by motor coach.
Use online resources to find licensed and reputable European tour operators. The European Tour Operators Association offers an online database of its members. These tour operators offer a range of services, from wine tours to castle tours.
Look for tour groups in this country that organize European travels. Examples of clubs that arrange tours include the Sierra Club where hiking and bike tours are offered to club members, university alumni associations where you go on organized European trips with other school alumni, and church group outings.
Use websites designed to organize travel groups of like-minded people. Examples of these include the International Gay & Lesbian Travel Association or the Women's Travel Club---a women only travel club.