Add list of users from file by UPN and add to Global Group

$ADS_PROPERTY_APPEND = 3
$userOU = [ADSI] "LDAP://cn=myUser,ou=Users,dc=contoso,dc=com"
$groupOU = [ADSI] "LDAP://cn=myGroup,ou=Users,dc=contoso,dc=com"$groupOU.putEx($ADS_PROPERTY_APPEND, "member", @($userOU.distinguishedname))
$groupOU.SetInfo()


I'm new with Powershell so bear with me.... I found this script and need to modify this script to take a list of users
take a TXT file with a list emails "User Principal Name" and add these users to a global group.

I have a list of 15,000 users so this is why I'm trying to use ADSI. I also need to factor in 
if the users exists in the global group already, skip the user.