Ok, apparently "IM or audio conversations with three or more people" are "not available with Skype* users" according to https://suppont.office.com/en-ca/article

Ok, apparently "IM or audio conversations with three or more people" are "not available with Skype* users" according to https://suppont.office.com/en-ca/article/Let-Skype-for-Business-Online-users-communicate-with-external-Skype-for-Business-or-Skype-contacts-b414873a-0059-4cd5-aea1-e5d0857dbc94 https://suppont.office.com/en-ca/article/Let-Skype-for-Busin... This makes it impossible then.