Hi @TMKramer ,
I'm glad to hear you solve the problem, if you have any issue about SharePoint, you are welcome to raise a ticket in this forum.
I would make a brief summary of this thread, You could click the "Accept Answer" button for this summary to close this thread, and this can make it easier for other community member's to see the useful information when reading this thread. Thanks for your understanding!:
Issue Symptom:
Trying to manually add a group name to a field in a Sharepoint 2013 workflow gives an error:
Current status:
The issue has been solved. Ignored the error/warning and just typed in the group name and hit ok. It worked!