Mostly just metadata and ensuring profiles are public. Joomla is pretty terrible with metadata management though and you may want to seek out a 3rd party Joomla extension at Joomla.org to allow custom metadata on a page by page basis. Otherwise the meta description specified in global configuration gets applied to every page. Alternatively you can leave that description blank and Google will use the first few bits of content on a users profile as part of their description. For example it'll have their name, online status, etc.. in the search results description.