While a posek should be consulted on this matter, my personal understanding and thoughts, based on what I have heard from prominent reliable people is that a shul and even a yachid cannot put someone in a situation where the person will either likely or definitely, (I cannot recall which, now) drive on Shabbos. More precisely, when dealing with someone who you know would drive on Shabbos, there has to be a practical and plausible alternative before you can invite him to your shul/home etc. on Shabbos.
Furthermore, to keep a shul parking lot open past the start of Shabbos, in a case where people will likely use it, seems irresponsible and a chilul Hashem. If they want to drive, they’ll find a way without you opening a parking lot for them as long as you do so with love and sincerity, not holier-than-thou nonsense.
A conservative Jew once mentioned to me in conversation that he lives too far from shul so he drives because how could he not pray to G-d on Shabbos just because he lives too far away? I was so thrown by this line of thinking that I regret I didn’t have an answer for him at the time that would not be CH”V embarrassing or offensive but still convey the fallacy of this argument.
The obvious flaw in that argument is that it is forbidden to drive on Shabbos. Prayer, however, can be done at home, too, even if this is not as preferable as praying in the synagogue. But, again, driving is absolutely forbidden barring pikuach nefesh et al reasons, so there is no justification for ASSISTING someone in doing so, CH”V. If someone parks around the corner then that’s his business, so to speak, but that doesn’t mean you leave the parking lot open to welcome the behavior, CH”V.
As with any mitzvah, the ends do NOT justify the means, and the future potential shemiras mitzvos is wonderful, but you can’t help him be mechalel shabbos to attain that wonderful goal. Let Hashem take care of His children while you work with them only within the framework of His Torah. Hashem is looking out for all His children and obviously does not want nor need you to assist in Chilul Shabbos.