Hi Nemanja,
I am the develper of the BigBlueButton integration with Moodle. Unfortunately the BigBlueButtonBN plugin doesn't have support for guest users. As for version 2.1.5
(2016051909) It is required for the users to be authenticated for accessing the activity/room, and this has been the case since the very fist version that run on Moodle 1.8.
The bug may be that the role guest should not be listed in the participant-role list. That is making people to think that guest users are supported.
The good new is that as the feaure has been requested quite a few times in the last year and was added to the roadmap during the summer 2016, it will be included in the next version that is in development right now.
Keep in mind that it is an alpha version. I won't recommend you to use it for production
Funny fact. Just today after completing the merge of the two plugins (from now on users will only need BigBlueButtonBN for everything!) I started looking to the implementation of that feature :)
Thanks for your interest in BigBlueButton and stay tunned.
Regards,
Jesus Federico