The unit of sharing is the channel. You start with a personal channel [email protected] and can create any number of new channels ([email protected]). Each channel has followers, moderator etc that you can control access to (or just set it as public): https://buddycloud.org/wiki/Channels_Explained

Think of the architecture like your email provider: You can always grab your mail spool and find a new provider. buddycloud is not dissimilar - just grab your posts and followers from the API and import them on a new provider. We're make this easier soon but, the data and endpoints are all there to facilitate it.