How do I connect light fixture’s ground to supply wires?
If it was me I would go to to the electrical supply store and purchase one of this type of safety GND wire pigtail. 




Loosen the screw on the lamp mounting bracket and install the spade lug on the pigtail under this same screw. Re-tighten.
Cut off the end with the green screw and strip the wire insulation about 3/4 inch.
Remove the wire nut on the GND wires currently in the electrical box and then merge the new stripped end of the green pigtail into the bundle and re-bind the bungle of GNDs with a good quality wire nut of the appropriate size.