Grounds twisted without wire nuts
It's not a legal method now to not use a Listed connector, I don't know if it was ever technically legal, but the the NW US that was common practice back in that era. 

Wire connectors have specific combinations that are Listed for it's use, but the normal colors represent trade norms. A yellow would be too small, a red would be fine, a tan which is between the two is normally Listed for 5 #14's, but the 4 wires being pretwisted increases their diameter, and you will likely struggle to get it to work properly.

All grounds are suppose to be bonded together, a typical large blue wingnut is only listed for up to 6 #12's or #14's, often you will find more than that jammed in, but the proper method is to jumper two smaller connectors together, or use a push in connector with enough ports. Using a push in connected with previously twisted and bent wires sometimes proves challenging. It might be best to use a red on the existing bundle with a pigtail to the second smaller bunch, and add your new ground wire there.