How to create a distribution list in exchange that contains only external email addresses and make it available in the GAL

I have tried the following methods but still not having luck:

https://community.spiceworks.com/how_to/196-adding-external-emails-in-ad-distribution-groups

https://technet.microsoft.com/en-us/library/bb124384(v=exchg.80).aspx

Can someone please help?  This is for exchange 2007.  thanks!

capitan