I did my daughter's formal wedding last year. In our part of the country we do have guests. Each invitation I sent to a single friend of my daughter's had the interior envelope addressed Mr. X
and guest or Miss X
and guest. Some people brought guests and some did not.
My personal opinion is that if the bride's parents wish to invite guests, it is perfectly permissible to do so. Showing up uninvited, however, is not.
You can, if you want to, say that people can bring a guest. Usually, though, "guests" are for those in serious relationships. But the uninvited are a BIG no no.