Yes, count as in earl or thane. The Latin comes, comitatis, originally meant 'companion, escort' from the roots for com 'with' and eo, ire 'go'. (Whereas a companion is somebody you break bread with.) In the '70s, there was a group of right-wing types who argued that the power to police at the county level was not in the hands of any police force but in the people's right to form a posse commitatus, i.e., the posse of the old West. They were also investigated on tax evasion and domestic terrorism.

More info http://foia.fbi.gov/posse.htm