You can use the arguement:
/castsequence [target=player] Rejuvenation,,,,
/castsequence [target=party1] ,Rejuvenation,,,
/castsequence [target=party2] ,,Rejuvenation,,

Depending on the toon name, Party1 is six characters, which might be shorter.
Six times five is thiry, which is probably less characters then the combined letters of the five toons.

An addon like macaroon or supermacro (think that's its name), will allow more characters within one macro.

Or as Fur mentioned, you can create a click castsequence.

/Click MultiBarRightButton1
/Click MultiBarRightButton2
....

On Button 1
/castsequence [target=toonA] Rejuvenaton,,,,,

On Button 2
/castsequence [target=ToonB] Rejuvenation,,,,

You can even time the number of comma's, so your healer will be placing a Rejuvenation on each party member every 10 seconds or 60 seconds. If you want, you could include other heals, such as a Lifebloom every ?? seconds on the tank, in addition to the Rej's on party members.