If it was me I'd take the dumb approach and do this:
- If it's possible to add a stone to a bunch that turns it into a group and moves towards the player's goal (i.e. odd number of groups for player 1, even number for player 2), do it.
- Else, if it's possible to add a stone to a bunch that prevents it from turning it into a group that'd go towards the opponent's goal, do it.
- Else, just place a random stone and hope it moves forwards (may want to put it next to an already placed one if possible).
Yeah, that's really dumb but I imagine it could end up getting reasonably far, especially given how easy it's for the AI to see those first two conditions. May want to keep this as an "easy" AI if you can find something better =P
EDIT: fixed the second step (derp)