Hi. From my life experience, I would say "recruitment dinner". They identified as Mormons on first contact. Now, they learned you are a 'fallen' JW - this is when the new fundamentalist friend will jump to SAVE you! I've had it happen more than once.
Since these people are your physical neighbours, I will quote my father "good fences make good neighours". The invitation has been accepted. By all means, attend and enjoy. As another poster said, they are like the JWs, and most likely very pleasant folk. Let them pitch their religious talk, but like most non-religious people, don't take the bait.
Be casual in your non-belief, as if it is normal. IT IS! :D Leave with a smile, after you have, by your lack of interest in reilgious discussion, dispelled any fantasies they have about converting or saving you.
If you feel you must reciprocate the dinner invitation as a neighbourly gesture, do so. Once. Then, remain as that 'friendly family next door' who never really has the time to socialize. Once they realize you are not interested in Religion and God, they will most likely stay to themselves anyhow.
Luck!
tal