I seem to remember that a lot of people nail a tuna can to the wall of the coop and fill it with oyster shells or grit, and the birds will take what they need.
I don't do grit - mine free-range and have lots of grit in their run. But for oyster shells, I put a heavy cat bowl next to the feeder with the layer pellets, and the layers will eat it at will. At least, I assume they're eating it... The level goes down, and they're not making a mess on the floor of the coop with it! The bowl is tapered on the sides and has never tipped. (I use the same kind of bowl for yogurt and they've never tipped it.)
Before I got a mixed-age flock, I added the crushed shells directly to the feed. Often I added crushed egg shells as well. But when we got some pullets, I wound up with two feeders - one with layer pellets and one with pullet crumbles. Since almost ALL of the birds like the pullet feed more, I had to add the bowl of shells. I've never seen a pullet eating from it, not once.