Hi Donna,
Lots of ways to get hold of the Group ID. If it's just one group ID you are looking for, the quickest and easiest way is to go to Setup>>Groups, then click on the group name to be taken to the groups landing page. The ID of the group that you have clicked on will now appear in the URL of your browser i.e. https://yourdomain.my.workfront.com/group/abc123/members
You can then simply copy and paste the ID element of the url (abc123 in my example above) into your spreadsheet.
Best Regards,
Rich.