Catalyzes the formation of apha-1,4 glycosidic bonds adding glucose residue from UDPG to the growing chain of glycogen. [(1->4)-alpha-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + H(+) + UDP Optimum pH is between 7.5 and 8.0. Glycan biosynthesis; glycogen biosynthesis. Belongs to the glycosyltransferase 3 family.