If you had Pending purchases before the membership program began, they will still be valid after you become a member.
However, if, after activating your Membership your account becomes "Not Activated" again, your shopping and referral “Pending” SG from after when the Membership Program began will become “N/A (Non-member).” Please see this article for further details: After Activating My Membership, What Happens if My Membership Becomes Not Activated Again?