Noob question Users and Groups

Fairly new to server admin, so please keep that in mind 😉

I thought I understood Users and Groups but something is not quite working right.

User 1 - member of Group 1 & Group 2

User 2 - member of Group 2

Share 1 - for Group 1

Share 2 - for Group 2

User 2 creates a new file in Share 2. User 2 can read and write (update) that file at will.

User 1 reads the file, makes and change and tries to save it but gets a permission denied. Why? Since they are both members of the same group I had thought they would both be able to create, read, update all the files in that share. Am I wrong, and if so, how do I fix it so that any one in the group can read/update all files regardless of who created them?

Server 10.8.2