You can add users to a group in bulk by using the Groups Spreadsheet. This method comes in handy if you have a large list of email addresses that you are trying to add at once.
Navigate to Directory > Groups > Create and then select "Get started" under "Bulk" followed by 'Blank Template' to export a blank spreadsheet.
In Column A enter UPDATE and in column B enter the group's email address.
For each member, add a line with the group's details from Step 2 and input the user's email address in column AH.
Navigate to Directory > Groups > Update and select the spreadsheet from the dropdown
Select 'Email me when this task is complete' to receive an email notification once it has been completed