Came right with this:

AddRole method:

public bool addRoleToUser(UserInfo user, string roleName, DateTime expiry)

        {

       bool rc = false;

       RoleInfo newRole = roleController.GetRoleByName(user.PortalID, roleName);

       if (newRole != null && user != null)

       {

       rc = user.IsInRole(roleName);

                roleController.AddUserRole(user.PortalID, user.UserID, newRole.RoleID, DateTime.MinValue, expiry);

       user = UserController.GetUserById(user.PortalID, user.UserID);

       rc = user.IsInRole(roleName);

       }

       return rc;

        }

Create user and add to role:

private void ApproveFor(WholesaleClient client)

        {

            client.IsVerified = true;

            var userInfo = GenerateUserInfo(client);

            var createStatus = membershipProvider.CreateUser(ref userInfo);

            if (createStatus == UserCreateStatus.InvalidPassword)

                throw new Exception("Invalid password");

            //save back to table with IsVerified set

            if (createStatus == UserCreateStatus.Success)

            {

                var roleName = "Wholesale Client";

                addRoleToUser(userInfo, roleName, DateTime.MaxValue);

                clientRepository.Update(client);

                return;   

            }

            throw new Exception("Error occurred");

        }