To protected an INDICO category or event, we should be sure that the person that need to access has a valid Indico account.

To create an external account he/she should go to the page : 

https://auth.cern.ch/

and click on External email - Guest access

then click on "New user? Register" 

and fill the form using the same address we will add in indico or in the egroup that will protect indico