Note: To keep the information in your company safe, it's best to to have separate workspaces set up for different clients with different external user accounts.  Users that share the same space may accidentally share information with each other.

External users have a minimum amount of access on the network, and can only be invited into the network by other Moderators on the workspace.  While they can search across the workspaces they have access to, by using the persistent search box in the upper right corner, they cannot browse the network for new workspaces to join. 

The content that a guest user has access to is completely dependent on the Page and Folder Security settings.  Each workspace is flexible and guest users can have access to all of the pages and files or none of them.  It is always up to the Moderators of individual workspaces to set the appropriate security on pages and files. 

Support can help you to understand the security model, but Moderators of workspaces are the only ones empowered to change the folder or page security of any page or file in your workspace.

To control the frequency of their notifications, all users need to click on their logged in name in the network bar, and access their profile.

If an external user does not appear in workspace's users list, then they do not have access to any content on that workspace, nor will they ever discover the workspace exists.  External user accounts only have access to the workspaces they are explicitly added to.  Once they have access to the space, additional page and folder security can be added to hide content from them, provided they do not have Moderator permission on the space.