SharePoint hub sites new in Office 365

Hi Katrin Weixel & seebee - it is possible to connect an existing (classic) team site to a hub site without associating it to a new Office 365 group - the caveat being that you need to be using a modern home page on the classic team site so that the modern Site information pane shows, where it is then possible to select the desired hub site you have permissions to join. On any classic team site, it's possible to create a modern page, place the desired web parts in the desired layouts per section(s), and then from the Pages library (Site contents > Pages document library), you simply select your new modern page and "make it the home page."

And seebee - no, you cannot join a subsite directly, you join the whole site collection. If need be, you can migrate content from a subsite into it's own new site collection if you need it to be a unique site joined to a hub site. However, if you have subsites, they will remain under the site collection as is when the site collection itself is joined to a hub site (we don't change hierarchy within the site collection).