A Clench is a local group of like-minded SubGenii. In more recent years, with the advent of teh Internets there are exclusively online Clenches as well. Clenches are local churches which belong to a larger religion, in this case the Church of the SubGenius. However, Clenches don't usually have buildings such as temples, mosques, synagogues, etc.
This wiki, the SubGenius Wikia Clench, is actually an example of an online Clench, as is the Ministry of Slack. A Clench can have anywhere from zero to an infinite number of SubGenii in it; plenty of Clenches just have one SubGenius in them. After all, since every SubGenius is an ordained minister and a typical local church only needs one minister, one SubGenius is all you need to form a Clench. Clenches with zero members typically result when everyone leaves an existing Clench, for instance if they are Ruptured away or if they leave the Church in schisms. The Clenches with zero members are referred to as the "inactive" Clenches.
There is a VERY incomplete list of Clenches at subgenius.com.
Here is a recruitment video for a Clench known as S.L.A.K. (SubGenius League of Ass Kickers):