The nature of a conference where delegates have to register in a private capacity means that there can be an assumption that no members of the public are present.
However, a conference audience would still be considered as 'public broadcast' and as such the organiser should arrange a MusicLicence to respect the copyright holders work if (for example) music is used between presenters etc.
If part of the event is streamed live via Youtube, this is broadcasting the content and the nature of the audience is no longer relevant. If the Youtube channel is monetized, then the organiser will be making money by broadcasting copyrighted content.
This requires licensing or may be forbidden, and Youtube may block content or close the channel/account if not adhered to according to it's terms & conditions of use. Even if licenses and permissions have been obtained, Youtube may not allow broadcasting of some content.