Not the exact same situation, but we have a trip coming up in which some family members are arriving on April 15 and others on April 20.
The FP window for the April 15 arrival opened this morning, and my daughter was able to include those of us arriving on the 20th as part her FP party, even though our window won't open until Sunday.
Based on that, I would guess that someone with a resort reservation might (should?) be able to include anyone in their Friends and Family Group with valid park tickets in their party for making FPs.