If Old Testament saints were saved by grace through faith in Christ (I assume general agreement on this), how were they then not members of the body of Christ? What "body" are they members of?
Your premise "saved by grace through faith in Christ" has a familiar ring to it; however it is hard to find a basis in scripture for its application to the OT saint.
If it were true, then surely one of the prophets would have told the Israelites, when they were straying, to get back to their faith in Christ.
Can this be found anywhere in the OT?
All we have to go on is what the Book says.
The straying Israelites were told to get back to worshipping God by doing the things God had told them to do which were the works of the law.
Did they keep the law? The Bible says they did.
Keeping the law is not the same as not sinning.
One cannot spread the blood before it was shed.
The OT saint was "saved" from hell by doing the things he/she was instructed to do by God; we see even father Abraham not in heaven (Luke 16) before the blood was spread.
What "body" are they members of?
John the Baptist referrs to himself as a "friend" of the Bridegroom, not as member of the Bride.