Employer Groups help you to group together your employers. You can create as many groups as you like and an employer can be in many groups, but at least one group must be specified as the 'Primary Group' for the employer.
You can use them however you like - maybe group employers by Account Manager, or Location or Client Type - it's totally up to you.
Groups as unique to you as a user. When you give others access to employers they wont see any of your group information but they are able to set up their own groups.
Clicking it will take you to the page where you can set up groups and edit existing groups
When viewing the employer details page, click the 'More' menu option at the far-right of the page and choose 'Groups'.
From here you can add set the primary group for the employer as well as select additional groups.
If you have groups set up and employers have been added to groups then on the Employer Dashboard page to you'll see an option to filter the list by group.
When you download your billing data to a CSV file then the primary group for the employer is included in the data.