There have been reports of "Zoom-Bombing/Gate-Crashing" in the news or you have seen unexpected attendees in your own meetings.
You can take some preventative steps to make sure there is no disruption in your virtual meeting or instruction.

Zoom has released guidance for users who want to protect themselves from Zoom-bombing.

For public meetings, Zoom reminds users that when you share your meeting link on social media or other public forums, that makes your event public, and this means that anyone with the link can join the meeting.

Zoom also suggests users avoid starting public events using their Personal Meeting ID (PMI) because your PMI is basically one continuous meeting and you don't want others crashing your personal virtual space after the your meeting is over.

Zoom suggests generating random meeting IDs. Zoom also suggests using the Waiting Room feature, which allows hosts to control who enters the meeting.

Zoom also offers numerous suggestions for managing participants.

Use a password on your meetings that you change frequently and share only with the scheduled group of meeting attendees.
If possible don't use the same meeting ID between Administrative or Confidential meetings and Classroom meetings.

• Allow only signed-in users to join
• Lock the meeting
• Remove unwanted or disruptive participants
• Prevent removed participants from rejoining
• Turn off file transfer
• Turn off annotation
• Prevent participants from screen sharing
• Put participants on hold
• Disable video
• Mute participants
• Disable private chat

 

From Zoom.us blog regarding Zoom Bombing:
https://blog.zoom.us/wordpress/2020/04/01/a-message-to-our-users/
===================================================================================

Zoom bombing Prevention Cheat Sheet

 

• For education users we:
  • Rolled out a guide for administrators on setting up a virtual classroom. 
  • Set up a guide on how to better secure their virtual classrooms. 
  • Set up a dedicated K-12 privacy policy.
  • Changed the settings for education users enrolled in our K-12 program so virtual waiting rooms are on by default.
  • Changed the settings for education users enrolled in our K-12 program so that teachers by default are the only ones who can share content in class.
• On April 1, we:
  • Published a blog to clarify the facts around encryption on our platform – acknowledging and apologizing for the confusion.
  • Permanently removed the attendee attention tracker feature. (updated 4/2 to clarify that it’s permanently removed)
  • Released fixes for both Mac-related issues raised by Patrick Wardle.
  • Released a fix for the UNC link issue.
  • Permanently removed the LinkedIn Sales Navigator app after identifying unnecessary data disclosure by the feature. (updated 4/2 to clarify that it’s permanently removed)