I am wondering if there is a way to determine the user entity type (SPPrincipalType) in a Nintex workflow. If somebody could help me I would greatly appreciate it.

Up to now I could not find a solution. Some sites I looked into: http://blog.bonzai-intranet.com/analysthq/2011/01/how-to-identify-if-sharepoint-person-or-group-field-entry-is-user-or-group/

http://blog.qumsieh.ca/2010/11/04/how-to-determine-entity-type-in-peopleeditor-control/

https://social.msdn.microsoft.com/Forums/en-US/ccbb0053-3ec6-4e71-aac1-ab7af0ff6086/how-to-check-the-given-name-in-people-picker-control-is-user-name-or-group-in-share-point-2010?forum=sharepointdevelopmentprevious

In the Nintex Forms People Control under Advanced (see the attachment) I could find out that there you can include the user entities like "User",  "Security Group", "SharePoint Group".