Translation:Are you coming over?
OK, so "eljossz" means that you are coming along, or come over. "Eljossz velem?" Are you coming (along) with me?" It could also mean "to come over, to visit." "Eljossz hozzank?" Are you coming over to our house?" Now, "atjossz" is more "come over" but it could also mean to "come across" a river, a street, etc. "Atmesz az utcan" You are crossing the street. "Atelem a haborut." I survive the war.
I'm just a beginner so I can't really tell but, come 'across' for at- seems like a good distinction. All the uses of at- so far seem to involve crossing a barrier, getting over an obstacle (real or metaphorical) whereas el- seems like just coming (or going) to a location from another.