If you have graphene, this is a single atomic layer material. Imho, there is no sensible definition of "unit volume" in this case.
If you have the weight of the carbon fraction of your sample (and guess you have because you have a result in emu/g) then you can readily compute the number of atoms in your sample from the molar weight. You can then still decide whether you report the spin density in Bohr magnetons per atom or whether you would insist in defining some atomic volume and then report in Bohr magnetons per unit volume.
Btw., one Bohr magneton is 9.274E-24 A/m^2. One emu is 1E-4 A/m^2 (??? I have never used this unit myself)
uB/f.u= M(emu/gm)*M.Wt/(bohr magneton* Avagadronumber). take care of units while substituting. as for as my knowledge this is general way to calculate emu/gm to uB/f.u. i dont know whether it is applicable to graphene or not.