Hi, @PM1 ,

To answer the first part of your question regarding the authentication, AAD is not the only method of authenticating users. Keberos can also be used, how K2 authenticates users depending on how it was set up.

To answer the second part of your question, naturally if a user is not part of an AAD, they will not have access to K2.

You can also have a look at the following links:
https://help.nintex.com/en-US/k2five/ICG/Current/default.htm#Configure/SF/MultiAuthAAD.htm?TocPath=Configure%257CK2%2520Site%257C_____2

https://help.nintex.com/en-US/K2blackpearl/ICG/current/Content/Plan/Authentication/Auth_Claims.htm

Hope this helps.

Regards,
 

Kevin