You can use the following Power Shell script to Add a User to Group.
# Authr: Gowtham Rajamanickam
# Title:Add User to a Group - PowerShell Script
#Version:: 1.0
Import-module ActiveDirectory
Import-CSV "C:\Scripts\Users.csv" | % {
Add-ADGroupMember -Identity AGNEWGROUP -Member $_.UserName
}
Import-CSV "C:\Scripts\Users.csv" | % {
Add-ADGroupMember -Identity AGNEWGROUP -Member $_.UserName
}