Beta-1,3-N-acetylglucosaminyltransferase that plays a role in the elongation of specific branch structures of multiantennary N-glycans. Has strong activity towards tetraantennary N-glycans and 2,6 triantennary glycans (By similarity). Protein modification; protein glycosylation. Interacts with B3GNT2; this interaction greatly increases B3GNT2 catalytic activity, independently of B3GNT8 enzymatic activity. Belongs to the glycosyltransferase 31 family.