After the 100 realised that there was more threatening their survival than the elements, they formed their own army, of which Bellamy is the leader. To help keep the delinquents under control, Bellamy 'hired' Nathan Miller as his Lieutenant.
Bellamy's militia is made up of teens of varying ages, genders, and races, and their endgame is simply to live. The militia is probably better suited to protect their surviving people as opposed to the Guards from the ark, because they have much more experience on the ground, allies, and presumably, there are more of them. The exact number of living members is currently unknown.
If you locate a character who's occupation lists them as being a member and they're not already listed here, please feel free to add them to the category!