The default attributes that Get-ADGroupMember returns are fairly basic.
distinguishedName, name, objectClass, objectGUID, SamAccountName and SID.
If you want more detailed user information, you will need to pipe the result into Get-ADUser.
Get-ADUser itself only shows a few attributes by default.
However GivenName and SurName are included in the default attributes.
If you want other attributes, you will need to include them by using -properties.
You can then pipe the results of this into Select-Object to control which attributes you want returned, dropping any extraneous ones.
Get-ADGroupMember -identity $group |
Get-ADUser |
Select-Object GivenName, SurName |
Export-CSV c:\members.csv -NoTypeInformation
# version 2 - how to return non default attributes like displayName and mail.
Get-ADGroupMember -identity $group |
Get-ADUser -properties displayName, mail |
Select-Object displayName, mail |
Export-CSV c:\members.csv -NoTypeInformation
# version 3, want the group name included in that
Get-ADGroupMember -identity $group |
Get-ADUser |
Select-Object GivenName, SurName, @{name="group";expression={$group}} |
Export-CSV c:\members.csv -NoTypeInformation