I am creating a group named MSProject01. Then create a txt file in the format below.
User
user01
user02
user03
Run the Powershell command
Import-csv "filename.csv" | %{ add-adgroupmember "groupname" -member $_.Users }
You can run the command below to see how many members are there in the group
(Get-ADGroup MSProject01 -Properties *).member.count