This script will export a .CSV for each group in an OU. The CSV will be named the same as the group name.
Get-Group -OrganizationalUnit "Some OU" | ForEach-Object { Get-Group -Identity $_.Name | ForEach { $_.Members } | Get-User | Export-Csv ($_.Name + '.csv') }
Remember this script is designed to run on one line. Remove the linebreaks.