In order to find collections shared within a Community where you are a member, go to the People tab of your account. On the left side of the page, you will see People Options. Clicking on Your Communities will display the communities where you are a member.
Find the Community with the collections you wish to review. On the right side of the page in the community lane, you will see two links, one for Reviews and one for Collections. The number in the blue circle indicates how many of each there are. Click on the number in the blue circle next to Collections:
Currently, you cannot add titles to a shared collection. You will need to copy the community to your own account. To do so, click on Other Actions and then Select All and Create Collection:
You can then name the collection. You will then be able to add any titles you want. You can then share the collection back to the community page.