The correct sentence with a proper compound subject is:
Margo and they prefer swimming over running.
However, it should be noted that while this sentence is grammatically correct in terms of using a compound subject, it may still sound awkward. A more natural phrasing might be "Margo and I prefer swimming over running" or "Margo and they prefer swimming."
The other sentences have grammatical issues:
- "His father and them" should be "His father and they."
- "My brother and me" should be "My brother and I."
- "Elissa and her" should be "Elissa and she."