I just want to share a bug with you folks that I reported to LL, because it can really throw you off if you don't expect it.
It looks like in the XUI changes of 1.12.1, the "share with group" checkbox got disconnected from reality and no longer reflects whether an object is actually shared. Example:
1. Create two prims.
2. On one, click "share with group".
3. Look at the second in edit mode.
4. Unclick the "share with group" box on the second.
5. Look at the first.
You'll see that the "share with group" box seems to simply stay how you set it, no matter which object you have selected. This means that you can't tell whether your own objects are shared. It means that if you check the box once during a session, it may seem like other objects you edit later are shared, but your group members can't actually edit them. It means that your fellow group member can't tell whether your object is shared, except that they still do see the "you can modify this object" message.