Better yet, read the publications of the groups themselves. According to John Bennett Shaw, it's two Sherlockians, a copy of the Canon, and a bottle. In a pinch, he says, you can dispense with one of the Sherlockians. This is exactly what Helen Yuhasova had to do in 1946. When she couldn't locate any other interested women, she formed The Solitary Cyclist of Washington D.C. and met at unspecified intervals with herself.
Most groups do contain more than two people.
In fact most contain more people than their name implies. The Fifth Northumberland Fusiliers, however, have never quite equalled the full strength of that illustrious regiment. Some groups start with a chance meeting, two people discovering a common interest in Holmes. A teacher's question to an elementary school class led to the founding of The Shadows of the Elms. Sometimes it begins with an ad in a newspaper or magazine.