I thought this macro would be easy...

[Ghallo]

Maxion's solution will work because the spells aren't "in position" yet.

With:
/castsequence [target=player]reset=5 ,,,,Rejuve
/castsequence [target=party1]reset=5 ,,,Rejuve
/castsequence [target=party2]reset=5 ,,Rejuve
/castsequence [target=party3]reset=5 ,Rejuve
/castsequence [target=party4] Rejuve

The first 4 don't advance because the spells don't match the spell that cast successfully.

Personally, I'd have this go "backwards" and modify it like this:
/castsequence [target=party4]reset=5 ,,,,Rejuve,nil
/castsequence [target=party3]reset=5 ,,,Rejuve,nil
/castsequence [target=party3]reset=5 ,,Rejuve,nil
/castsequence [target=player]reset=5 ,Rejuve,nil
/castsequence [target=party1] Rejuve

So that if you spam it, they will always cast on the "leader" if you are spamming it "too much"

No HKN, IS, or Keyclone needed. Just put this on the same key on all toons.
Edit: As stated by Maxion - you need to add leading commas per your spam rate.