Joins adenosylcobinamide-GDP and alpha-ribazole to generate adenosylcobalamin (Ado-cobalamin). Also synthesizes adenosylcobalamin 5'-phosphate from adenosylcobinamide-GDP and alpha-ribazole 5'-phosphate. adenosylcob(III)inamide-GDP + alpha-ribazole = adenosylcob(III)alamin + GMP + H(+) adenosylcob(III)inamide-GDP + alpha-ribazole 5'-phosphate = adenosylcob(III)alamin 5'-phosphate + GMP + H(+) Cofactor biosynthesis; adenosylcobalamin biosynthesis; adenosylcobalamin from cob(II)yrinate a,c-diamide: step 7/7. Belongs to the CobS family.