An anonymous user is a user who does not need to log in to Bonita BPM before accessing a process form. An anonymous user is always be the initiator of a process. A form that is used by anonymous users must have the autologin property set. You can configure autologin for a form when you define it in Bonita BPM Studio, and you can specify autologin in the form URL.
All access to the Bonita BPM Engine needs a username and password, so to enable anonymous users, you need a user entry in your Bonita BPM, like a shared "guest" account for visitors. You can manage this user in Bonita BPM Portal, just like any other user in the organization.