Attaches N-acetyl-alpha-D-glucosamine residues to poly(RboP)-wall teichoic acids (WTAs). 4-O-[(D-ribitylphospho)(n)-di{(2R)-glycerylphospho}]-N-acetyl-beta-D-mannosaminyl-(1->4)-N-acetyl-alpha-D-glucosaminyl di-trans,octa-cis-undecaprenyl diphosphate + n UDP-N-acetyl-alpha-D-glucosamine = 4-O-([2-N-acetyl-alpha-D-glucosaminyl-1-D-ribitylphospho](n)-di{[2R]-1-glycerylphospho})-N-acetyl-beta-D-mannosaminyl-(1->4)-N-acetyl-alpha-D-glucosaminyl di-trans,octa-cis-undecaprenyl diphosphate + n H(+) + n UDP kcat is 126 min(-1). Cell wall biogenesis; poly(ribitol phosphate) teichoic acid biosynthesis. Homotrimer. Assembles into a propeller-like homotrimer in which each blade contains a GT-B-type glycosyltransferase domain with a typical Rossmann fold. Assembles into a trimer using a novel trimerization domain, termed the HUB domain. Inactivation of the gene leads to resistance to serogroup B phages. Disruption leads to altered WTA that lacks the alpha-GlcNAc residues. Mutant exhibits no major changes in growth kinetics, microscopic appearance or antibiotic susceptibility. Belongs to the glycosyltransferase group 1 family.