There's a couple of ways they could do this. First, I haven't heard anything on whether Microsoft has provided this in the XNA tools, but they would need to provide developers a means to do achievements in the tools. This should include hooks to pop up the achievement, use an icon, associate a name, description and point value. Since Community Games are basically indie-developed XBLA games, they should use the same guidelines: 12 achievements worth a maximum 200 points.
Then are couple ways they could go next:
1) They could not allow those achievements and points to even show up on your Gamer Card and Gamerscore. The only way people would know you you got them is if you told them. The reason for having the achievements is personal gratification, but the developer would have them in place if the game ever got picked up to be a full-fledged XBLA game.
2) They could allow them to show up on your Gamer Card and Gamerscore, but they would be worth only zero points. I could live with this, but I still think the next option is much better.
3) They could create a separate Community Games Gamercard and/or Gamerscore that just shows those achievements and points. This would be the best, since it doesn't contaminate your true Gamerscore and people could go hog wild if they wanted.
AzBat