Invitations

It depends on how you plan to print the invitation.  4 up puts document page size at 11x17 with enough space for trim marks and bleed ( if any ).  Quantity also depends on how you plan to print the invitations, that and the paper you plan to use.  Does it fold?  Will it need to be scored?  All of those varaibles determine what type of page setup you need to plan for.

An additional suggestion:

"Effect>Distort & Transform>Transform" is a good solution to duplicate. Another is to save your artwork as a "Symbol" then duplicate the symbol into your multi/up template. Make any changes to the Symbol using the "Edit Symbol" button or by double-clicking the Symbol icon in the Symbol Panel. Any changes will be updated on all the Symbol "instances".

A standard page size is 8.5"x11", so if you're using standard pages, you won't be able to fit four 5"x7" images on the same page.  You could fit two though, and then trim your margins after the fact.  Usually when I want to set up something specific like that, I'll place my sized artwork/text onto a page sized artboard and print an export with the borderless setting (if you have that, otherwise just make sure you're printing at 100% size so that your work isn't being shrunk to fit more easily on the page).  Hopefully that makes sense.

Julie,

As I (mis)understand it, with your Artboard size set to the paper size, you can just Group the 5x7 artwork, then scale down if needed (depending on your unit and paper size), then place it at the lower or upper left with the desired outer margins and use Effect>Distort & Transform>Transform twice with the right Move values horizontal and vertical to also get the desired outer margins for the copies with the inner distances being twice the margins so all four can be cut with the desired margins.