Involved in the biosynthesis of phosphatidyl-myo-inositol mannosides (PIM) which are early precursors in the biosynthesis of lipomannans (LM) and lipoarabinomannans (LAM). Catalyzes the addition of a mannosyl residue from GDP-D-mannose (GDP-Man) to the position 2 of the carrier lipid phosphatidyl-myo-inositol (PI) to generate a phosphatidyl-myo-inositol bearing an alpha-1,2-linked mannose residue (PIM1). a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol) + GDP-alpha-D-mannose = a 1,2-diacyl-sn-glycero-3-phospho-[alpha-D-mannopyranosyl-(1<->6)-D-myo-inositol] + GDP + H(+) Phospholipid metabolism; phosphatidylinositol metabolism. Monomer. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.