When you are in the Group, click at the top where it says THREAD TOOLS
then click on SUBSCRIBE TO THIS THREAD.
Then when you need to find it, go tour subscriptions and enter from there.
Alternatively, you can subscribe to the group itself. When you need to get there, click on Groups. Once at Groups, down the page a bit and on the right, it shows a list of all the Groups that you are subscribed to.