After creating a new User Group here is how you can assign all users to this group. See steps below:
Select the Setup portal.
In the left side menu you will select Resources located under the Schedule Setup blue bar.
Select User Setup from the top menu bar.
Once in the Resource Assignment window, you will select the magnifying glass icon in the top right corner. In the drop down list, select User Group. 
You can then select Grid Assignment from the tabs.
You will then click on the user group name that you created. This will assign all users to this group. Be sure to select the Save button at the top before closing out of this window.
