By default, if someone tries to check-in to a segment that they haven't registered to, they will automatically be added to the segment if there is space available. Check out this article on Segment Registration.
However, you can adjust this setting by tapping on the burger menu and tapping on Event Settings.
You'll see a setting for "Segment Auto Registration" - toggle this to "off" as shown in the screenshot below.
Now, when an Attendee checks in to a segment that they are not registered to, the Organizer has the option to check them in, or not.
To check in the Attendee, tap Register and Check-in or you can tap Close to to refuse them entry to the segment.
This will open a new pop-up where you can confirm the registration. Tap Yes or No.