Involved in the biosynthesis of phosphatidyl-myo-inositol mannosides (PIM) which are early precursors in the biosynthesis of lipomannans (LM) and lipoarabinomannans (LAM) (PubMed:19395496). Catalyzes the addition of a mannosyl residue from GDP-D-mannose (GDP-Man) to the position 6 of a phosphatidyl-myo-inositol bearing an alpha-1,2-linked mannose residue (PIM1) to generate phosphatidyl-myo-inositol bearing alpha-1,2- and alpha-1,6-linked mannose residues (Ac1PIM2) (PubMed:19395496). PimB also catalyzes the addition of a mannosyl residue from GDP-Man to the position 6 of phosphatidyl-myo-inositol bearing an acylated alpha-1,2-linked mannose residue (Ac1PIM1) to generate monoacylated phosphatidyl-myo-inositol bearing alpha-1,2- and alpha-1,6-linked mannose residues (Ac1PIM2) (By similarity). The addition of the second mannosyl residue by PimB preferentially occurs before the acylation of the mannosyl residue transferred by PimA (By similarity). Also able to transfer a mannosyl residue from GDP-Man to the position 6 of a phosphatidyl-myo-inositol (PI), but this reaction is very slow (By similarity). a 1,2-diacyl-sn-glycero-3-phospho-[alpha-D-mannopyranosyl-(1<->6)-D-myo-inositol] + GDP-alpha-D-mannose = a 2,6-O-bis(alpha-D-mannopyranosyl)-1-phosphatidyl-1D-myo-inositol + GDP + H(+) a 1,2-diacyl-sn-glycero-3-phospho-[alpha-D-6-acyl-mannopyranosyl-(1<->6)-D-myo-inositol] + GDP-alpha-D-mannose = 2-O-(alpha-D-mannosyl)-6-O-(6-O-acyl-alpha-D-mannosyl)-1-phosphatidyl-1D-myo-inositol + GDP + H(+) Phospholipid metabolism; phosphatidylinositol metabolism. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.