It has been my experience that unless one is part of the wedding party, all they have to be is at least not be in coveralls and a straw hat. In todays world "business casual" is probably alright, but a suit is absolutely not necessary. With what you described it is alright but if you are concerned, a simple sport jacket/blazer will be more than enough. If money is a problem they can be found, sometimes in almost new condition, at any good will type location. Then it becomes practically disposable or returned to that location so someone else can knock them dead at a wedding.
This is an old thread but people still get invited to weddings, so it is something that can remain current.