Just feed them regularly and they will multiply rapidly. The absolute best thing you can do for your lawn is to mow it to the right height. Take a sample cutting to the professionals at your local garden center, who should be able to identify the type of grass and recommend its ideal length. Many a lawn has been ruined by too close a cut. While I strongly recommend a chemical-free lawn, if you do use a fertilizer, make certain that it's not harmful to earthworms.
Some chemical fertilizers can increase soil acidity to levels intolerable to worms.
I'd advise conducting a soil test to see if a fertilizer is even necessary, and if so, in what amount. You may well be introducing chemicals into the environment needlessly. Finally, pull weeds out by hand or cut them out with a hoe, then save them for the worm pile. The earthworm garden is mulched with newly cut grass in summer and with lawn mower-chopped leaves in fall. The mulch, which provides the worms organic sustenance, should be thick enough to block sunlight and retain soil moisture, but thin enough to allow air to get through.