Can I run 4 #12 solid wires in 1/2 pvc / for 35 feet to a junction box and then run two more circuits out of there sharing the grounds

If you mean conductors in the conduit, not cable, then yes, you only need one ground for the entire conduit. This is sized according to the largest circuit conductor.
If ALL are #12 you are fine.

I couldn't locate current Code Book for exact number, did find this:
Mike Holt
Go down to "Free Stuff", then "wire fill calculations"
Then Choose EMT or PVC, and enter number of conductors, it will tell size of conduit.
Type in "8", it will say 1/2". if you say "9" instead, it will tell you to use 3/4"(this is for PVC)
8 is the maximum conductors in 1/2 PVC, 9 is max for 1/2" EMT.
I chose THHN for the Insulation Type.