As an administrator and event manager, when making a new event, you have the opportunity to update its registration settings. Some of the many registration settings include letting guests register (i.e., people without accounts and people who are only singing up episodically), letting minors (children and kids without an email and will be chaperoned by a parent), activating waitlists (which will automatically expose a mechanism to place people in waitlists and inform to register, whenever there is availability), enabling Group Registration (which will let team leads and individuals representing large groups to bulk register themselves and letting the admin reserve spots for them).
Note: Waitlisting, Guest Registration, Minors Registration and Group Registration are automatically turned off. You as the administrator will have to turn these on.
Once the above settings are applied, you will be able to see who is registered with the different various status (i.e., waitlist, or any other specific status related to the registration) via the "Manage Registrations" workflow ("events"->"registrations" or inside the specific event you can select "Manage Registrations":
Note: in the case Status is not displayed, you can "configure the layout" and select "status". By filtering by "waiting approval" you will see all the individuals that are int the waitlist status:
Note: An admin can always update the status of the registration record as "registered" to overfill the event and that will email the registrant informing them that they are now registered. If you don't manually add them this way, and if someone cancels, or you add more spots, they will automatically be added in the order they were added to the waitlist.
Additionally, if you need to manually add someone to the waitlist, you can do so via Manage Registrations -> Actions -> Waitlist: