Would this involve summing the DM particles within Group_R_Crit200, and if so, what is the mass of an individual DM particle?
Yes you could imagine creating a Group_M_Crit200Type in analogy to the other *Type fields available.
For the mass see MassTable of the data specifications.
When I load the group catalogue header using header = il.groupcat.loadHeader(), 'MassTable' isn't listed as a key.
This will only be in the snapshot headers.