If you just want it at the bottom of the page it'd be easier to just have it as a module. You really don't need AddThis or any other special plugin to share URLs on social sites. You can even share URLs on social sites without any JS libraries. We've done this using the below.
You'd add those links to a Custom HTML module or CB Content Module then place it on any page you want to allow sharing. No heavy JS libraries, etc.. Replace the "Share on ..." text with an image if you want them to be image links.
If you still want to insert directly on a groups page though you can use a Code action in CB Auto Actions on the gj_onAfterDisplayGroup trigger. You'd set Method to HTML and echo the results, which should show on the bottom of every group.