how can you guarantee that your visitor will return

If you invite him to come by the "invitation letter" and the immigration accepted,  you will be responsible for his staying and leaving.
If he don't leave the country for what kind of reason,  you get the problem. If he is involved by an accident, for example,  you need to pay all the costs (hospital,  recovering).

I assume you can get a 3bis from the local commune (or you may already have got ? ) which is kind of official declaration that you are a resident here and are responsible for the visit and return of the visitor.

Accidents and repatriation due to health reason are covered by a good travel insurance. Apart from that, there is little you can do to prevent an overstay.

Do I need a 3bis form for long term spouse visa?
My wife is Belgian born citizen and I am applying for a long term D spouse visa for permanently shifting to Belgium.
If yes, any idea how much time does it take to issue a 3bis form from the local commune.
Any information regarding this will be highly appreciated.

You dont need a 3bis for dependant VISA application.

To answer the next question, 3bis can be obtained in one day from commune.