create a distribution list and add them to that list. Then email the list.
Keep in mind that there is a size limit to outlook distribution lists. If there are a large number of people you are going to email, you may have to break it down into more than one list.
Another thing to note with the distribution group. You need to create Active Directory contacts for each external email address.
If this is an Outlook distribution group, no external contacts need to be created in AD. Outlook will be able to send successfully and that step is not at all necessary. Active Directory distribution Groups will need the AD contacts to be created, yes, but this question was about Outlook contacts, not Exchange AD contacts. Didn’t want to get you confused Marriot. The size limit pertains to Outlook distribution lists. Exchange AD DL’s have a limit which can be overcome by nesting groups inside the DL, but the Outlook DL cannot be nested to overcome this limit.