Hi PatsyHo,
The following steps address what you're describing specifically, and should provide a solution:
"In your email, either the list name or individual contact names are shown, depending on how Composing settings are set in Mail.
If the list name is shown but you want to see the individual contact names instead, click the arrow next to the list name, then choose Expand Group."
You can find these steps along with other information that may be helpful, here: Email a list or Smart List in Contacts on Mac – Apple Support
If you have followed the steps correctly and you still have the issue, let us know if you get any specific alerts or error messages when trying, as those may help us determine what might be causing the issue.
We also recommend restarting your Mac and keeping it updated as those steps assure you have cleared out any minor software issues and are running the most current version.
Shut down or restart your Mac – Apple Support
Update macOS on Mac - Apple Support
If you still have questions, reach out to Apple Support directly, as this may require closer guidance: Get Support
Cheers!