Leftovers are kosher, so to speak. No offense would be taken by bringing turkey and stuffing and pumpkin pie and all the fixin's over to your neighbors' house. Jehovah's Witnesses wouldn't equate eating leftovers with celebrating Thanksgiving, since they would not have been present at the meal itself. A traditional Thanksgiving feast on Thanksgiving day has religious and other overtones that may make your neighbors uncomfortable.
As for inviting them to the meal itself on the actual day of thanks, you can always ask. Whether or not they join you is a matter of a conscience for each Witness. Regardless of their decision, it is the rare Witness who will be offended by your invitation. Indeed, even if they decline, they'll still be honored that you wanted to include them in your celebration.
Jehovah's Witnesses celebrate no civil holidays and few Christian ones. Thanksgiving upsets them for several reasons. First, they are keenly aware that the day is rooted in ancient (and pagan) European harvest festivals. They also criticize the idea of reserving one particular day for giving thanks rather than encouraging spontaneous thanks every day of the year, which is the principle in Ephesians 5:12: In the name of Jesus Christ, "to give thanks always for all things to their God and Father."
They might cite biblical injunctions against drunkenness and gluttony, which many Jehovah's Witnesses associate with Thanksgiving. And just as Jehovah's Witnesses don't salute the American flag because they consider that to be a form of idolatry, they also object to the call by Sarah J. Hale to conflate religion and patriotism on Thanksgiving. Hale, who persuaded Abraham Lincoln to declare Thanksgiving a national holiday in 1863, wrote that on Thanksgiving, "every American" will "thrill his soul with the purest forms of patriotism and the deepest emotions of thankfulness for his religious enjoyments."
In 1969, the Witnesses' magazine, Awake!, compared Witnesses' abstention from celebrating Thanksgiving to early Christians' refusal to "join with the idolatrous Romans in observing Roman religious holidays."
All this isn't to say that Jehovah's Witnesses don't enjoy themselves on Thanksgiving. Like almost everyone else, they have the day off from work, and they might visit with friends and family or have a meal with them. But that's not intended as a way to give thanks, which Witnesses attempt to do every day of the year, including, of course--in their own way--on Thanksgiving.