GRANT is for granting permissions to a role, user or login, not for adding a role, user or login as a member of a role.
From BoL
sp_addrolemember (Transact-SQL)
Adds a database user, database role, Windows login, or Windows group to a database role in the current database.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability