Hi @johnjohnPter,
You can for example query the siteusers and check the PrincipalType property. 1 should be a user, 4 should be a security group.
Alternatively, you should also be able to use a Get user profile (v2) action. This action should only return user profiles and shouldn't be able to find a profile for a group (in that case it should error with the message user not found).
Below are both examples in one flow.
_api/web/siteusers?$filter=Email eq '@{triggerBody()['email']}'&$select=PrincipalType
