Copy Multi-User Field to Multi-Line Text Field via Function

Copy Multi-User Field to Multi-Line Text Field via Function

I need to include a multi-user field in our Won Opportunity email notifications. Unfortunately, multi-user fields are not available as merge tags. It's been suggested that I use a function via workflow to transform the multi-user list to a text field that I can use in the email template. This is fine by me and I'm comfortable with the workflow, but I'm not sure how to write this function. Any help here would be appreciated.