Add active directory group to users is a procedure that back then before the windows shell option, could be achieved using 3rd party modifying tools.
With windows PowerShell, the option to add active directory group to users can be done by using the Add-ADGroupMember cmdlet as follow:

Assuming we want to add Daniel, John and Goe to group Finance Team the powershell cmdlet as follow:
Add-ADGroupMember  -Identity Finance Team -Members Daniel, John ,Goe
-identity will hold the group name required to add users to.
-members will hold member names.

You may also emport users to CSV as described here.Then use this CSV file to apply to -ADGroupMember cmdlet as follow(Assuming  exported to file named: Myusers.csv):
Import-CSV “C:\MyUsers.csv” | % {
Add-ADGroupMember -Identity Finance Team -Member $_.UserName