Ever since they patched out being able to woohoo with the Reaper so many years ago, all you can really do with Grimmy in Sims 4 is flirt with him. I believe going to a date with him maybe possible, but that's only possible if Grim called you for the date because I don't remember seeing it pop-up. And yes, he may stop by often if he has an apartment key. In fact, he's a freeloader and hogs the computer every time he visits this one chick I have in this save (I used her to kill somebody through anger).