Adding existing users to your new Novell Vibe OnPrem site was part of the installation process, as described in Adding Vibe Users from Your LDAP Directory
in Basic Installation
in the Kablink Vibe OnPrem 3 Installation Guide.
However, as time passes, you need to add new users. Depending on the needs of your Vibe site, you might want to allow people to access the site even if they do not have Vibe user accounts in the Vibe system.