A Google Meet meeting can have a maximum of 250 participants and you can invite additional people to a meeting at any time.
You can share the details of the meeting you have started, so that anyone you send the details to can join via the Meet link:
Click on the Info button in the bottom-right corner of the screen
The meeting URL will display - click on Copy Joining Info
You can now send the URL to all additional guests via email or Hangouts Chat.
You can add specific people to the meeting, so that they receive an invitation in their Inbox, with a link to join.
Click on the People icon in the bottom-right corner of the screen
Click on the Add People link at the top of the people pane
Enter the name or email address of the person you wish to invite to the meeting
Click on the person you wish to invite
Click on Send email
The person you have invited will receive an email that contains a link to join the meeting.