You don't have to be invited to join any more. You can find contact details on provincial/grand lodge websites and email them, which is how my oh joined (after a work colleague got him interested.)
You don't have to be "affluent"ie they don't check your financial status before letting you in, but lodge membership fees add up and staying to dine after meetings costs on top of that. And in my opinion it gets a bit much when your oh is paying separate fees to be in craft lodge, chapter, knights Templar, rose croix etc, plus the dressing up clothes (cloak sword and silly hat for kt, for example) and the special bags to keep them in... (Do I sound bitter?
) I've had to give up attending the annual knights Templar provincial priory church service as I can't keep a straight face - you've never seen anything so funny as a bunch of grown men in full kt regalia prancing round with swords and banners.
Having typed up a ritual book for my oh (upon joining chapter he was handed a manuscript and told to make his own copy) the lodge proceedings are based on the biblical stories of building/rebuilding Solomon's temple with lots of symbolism and are supposed to teach masons how to live good lives.
None of the Masonic wives/partners I know are involved in the Eastern Star - we prefer to meet up in local pubs while our OH's play their funny games.