Groups allow you to add certain users together so that you don't have to tag or notify each user individually. For example, if my entire Human Resources team needs to be added to every submission I could add them all into an HR group and add them to a submission all at once.
To create groups:
Click on your name on the upper right hand corner. Choose Site Settings from the drop-down menu.
Once in the Site Settings menu, click on the Groups Tab.
Select Create Group and add the emails of the desired users and hit Save.
NOTE: The user must be already set up as a user before they can be added to a group.