You can share custom buttons from three areas:
When you create the button, there is a checkbox on the lower right marked, Share with the Group. Check it, and your entire group will see this button.
When you edit a button, you will see the same checkbox.
In Settings + Directories, there is now a Custom Button tab under the Specific to Individual tab. Your entire list of custom buttons, and their locations, are listed on this tab. Check the Share checkbox next to any custom button, and it is shared with everyone in your group. You can see only shared buttons if you click on the button labeled Show All Shared Buttons.