If you are recording the Google Meeting for documentation or as a reference for later it is always a good idea to wait until all the participants have entered the meeting.  Make sure to let them know you will be recording and that they are ok with it.

If you are making a video presentation, you don’t really need to add other participants unless they are necessary to the presentation.  This means that you can start the recording at any time. 

You can pause or stop the recording at any time during the meeting.