The Customer Group is used to group your Customers together so that you are able to apply security within that particular group. For example, if you have different branch offices, you may limit employees to which customer records they may access.
First you will need to create at least one Customer Group. You will find this in SedonaSetup > Customer Groups in the CM area.

Once the Customer Groups are created, I would recommend first adding your customers to the correct Customer Groups. This needs to be done by customer, so you would go into the Customer Setup by right clicking on the Customer Information and selecting Edit Customer Setup.

Once the Customer Setup window is open, locate the Customer Group field and select the Customer Group you wish to have the customer a part of.
Then to enable the Customer Groups option. This is found in SedonaSetup > Setup Processing in the AR area.

In the AR Setup Processing window, in the Other section, there is a checkbox called Customer Group Security by User. You will need to check this box, then enter the default customer group.
Once this is enabled, you will get a message informing you that each customer without a customer group will be automatically assigned to the default you select.

Once Activate Customer Group Security by User is enabled, go to the Users setup table in SedonaOffice.

After choosing a User, you will see that a new tab has been added to the User setup called Customer Groups.
This is where you will assign the groups you wish your users to have access to.