Participates in the formation of the lipid-linked precursor oligosaccharide for N-glycosylation. Involved in assembling the dolichol-pyrophosphate-GlcNAc(2)-Man(5) intermediate on the cytoplasmic surface of the ER (By similarity). GDP-alpha-D-mannose + N,N'-diacetylchitobiosyl diphosphodolichol = beta-D-Man-(1->4)-beta-D-GlcNAc-(1->4)-alpha-D-GlcNAc-diphosphodolichol + GDP + H(+) Protein modification; protein glycosylation. Belongs to the glycosyltransferase group 1 family.