How to determine if Created by exists in SP group

you will need to make a call to userGroup.asmx web service to get all the group members (GetUserCollectionFromGroup method) or to get all the groups which's the user is meber of (GetGroupCollectionFromUser method).

https://msdn.microsoft.com/en-us/library/users+and+groups.usergroup.getgroupcollectionfromuser%28system.string%29%28v=office.12%29

based on that you should be able to decide  which branch to continue.

Thank you for the reply. The link you provided generates a 404 error.

I used this method: https://wowsomuch.com/sharepoint/nintex-checking-if-a-user-is-a-member-of-a-sharepoint-group/

This appears to be exactly what I am looking for. However, the workflow errors at the Action Set with the following:

Failed to invoke web service. Error returned from server: <soap:Fault xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"><faultcode>soap:Server</faultcode><faultstring>Exception of type 'Microsoft.SharePoint.SoapServer.SoapServerException' was thrown.

seems to be something broken at MS side...

direct links to webservice methods work, but webservice main page from above doesn't

UserGroup.GetUserCollectionFromGroup Method (WebSvcUserGroup) 

UserGroup.GetUserCollectionFromWeb Method (Users and Groups) | Microsoft Docs 

Resolved. This article describes how to do this, step-by-step: https://wowsomuch.com/sharepoint/nintex-checking-if-a-user-is-a-member-of-a-sharepoint-group

Do you have any workaround same for sharepoint online? as this solution does not work for SP online.