Catalyzes the addition of a single glycerol phosphate residue to the prenoldiphosphate-linked disaccharide, as a primer for polymerisation by TagF. CDP-glycerol + N-acetyl-beta-D-mannosaminyl-(1->4)-N-acetyl-alpha-D-glucosaminyl di-trans,octa-cis-undecaprenyl diphosphate = 4-O-[(2R)-glycerylphospho]-N-acetyl-beta-D-mannosaminyl-(1->4)-N-acetyl-alpha-D-glucosaminyl di-trans,octa-cis-undecaprenyl diphosphate + CMP + H(+) Cell wall biogenesis; poly(glycerol phosphate) teichoic acid biosynthesis. Positively regulated by WalR. Mainly expressed during exponential growth and rapidly shut off as cells enter the stationary phase. Belongs to the CDP-glycerol glycerophosphotransferase family.