As a moderator, you can restrict new participants entrance during the session.

Go to Settings in the navigation bar.

A popup window will appear showing three categories, go to CLASS > Allow entry of new participants.

Turn it on to allow new participants to enter during the session.

Turn it off to prevent new participants from entering during the session.

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This will only affect participants.
Other moderators in the session can still enter during the session.
These settings have no effect in Rooms since all users become moderators there.