What is the proper way to

word placecards?
Mr.& Mrs. J. Smith
or Mr. & Mrs. John Smith??
or another way ?

and for people bringing guests??

I think...
Mr. and Mrs. John XXX

Mr. John XXX and Guest

I believe it is the same way the envelopes were addressed:

Mr. and Mrs. John Smith
Mr. John Smith and Guest
Mr. John Smith and Ms. Jane Doe

Mr. and Mrs. John Smith
Sara, Paul & Eric

I'm sure you can do first inital if wanted for the sake of saving space, or the '&' instead of 'and'

Married Couple:

Mr. & Mrs. John Smith

If they are bringing a guest:

Mr. John Smith and Miss Joan Doe

But if you don't know the guests name(which you should try as hard as possible to find it out) you just put:

Mr. John Smith and Guest