Involved in the biosynthesis of phosphatidyl-myo-inositol mannosides (PIM) which are early precursors in the biosynthesis of lipomannans (LM) and lipoarabinomannans (LAM) (PubMed:10531370, PubMed:19638342). 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:19638342). 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) (PubMed:19638342). The addition of the second mannosyl residue by PimB preferentially occurs before the acylation of the mannosyl residue transferred by PimA. 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 (PubMed:19638342). 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. Extended N-terminus.