Some instances of readyplatform are designed to integrate with your organisation's Active Directory in order to enable Single Sign-On (users don't need to log in with a username and password). In this case, all users are automatically added to the platform and will appear in all drop-down lists. (Some exceptions apply within specific webapps where users may need to be specifically invited even if they have an active account on the platform).
If your organisation is using Single Sign-On, then you may not see any buttons or links to invite others users. However, if your organisation's configuration allows for it, you can invite other users in two ways:
Everywhere on the core platform and within webapps where you are presented with a drop-down list and asked to select a user, there will also be a link provided that you can use to send an invitation if you cannot find the user you want in the list.