On the deposit page, you can choose to share your work with up to five groups. Note that you have to be a member of a group before you can share your work with its members.
Simply start typing the name of a group you are a member of, and the available groups will appear in a drop down list. Adding groups will give much more visibility to your work, as members of groups will be notified that your piece was shared with them and users will also see it on individual group pages. Not a member of many (or any) groups yet? Go to the group directory to join groups that are relevant to you or start your own group!