Hard core consists of pieces of broken stone that are used as a base on which to build roads. a the small group of people that are most active within a group or organization
You can refer to the members of a group who are the most committed to its activities or who are the most involved in them as a hard core of members or as the hard-core members. We've got a hard core of customers that have stood by us A hard-core group of right-wing senators had hoped to sway their colleagues