I'm assuming you're using a focused setup right?

If so, you'll want a macro like this:

/clearfocus
/focus [target=toon1, exists, nodead][target=toon2, exists, nodead] (repeated for however many toons you have - substituting the toon1,2, names for your toon names)

Bind this to a key, and press it whenever your main dies or you form the group. Essentially it means, if the first toon1 is in the group and alive, he becomes the focus, if he's not in the group, or he's dead, the second toon becomes the focus and so on.

Your assist macros then look like this:

/assist focus
/cast Lightning Bolt (or whatever spell/attack)

This is of course, as I said, assuming you're using a focused not a focus-less setup.