You can make your WordPress site users to login from only single device at one time. This not only increases the security of your WordPress site but you can stop your users from sharing their login credentials with others especially if you are running a paid membership website.
It is possible to show-hide categories based on your WordPress site user roles. The CaPa Protect is a free WordPress plugin lets allows you to hide or display categories based on user roles types like Editor, Author, Contributor etc. The plugin also provides you the feature of showing a message whenever a someone arrives without permission on a protected blog post of your website. Also you can simply hide everything and your user will not be able to know the existence of hidden post.
Now you can reserve places for unregistered members of your BuddyPress site by using their email addresses.