@DavidMWright The issue may be happening if they are joining an old or incorrect link to another or past meeting.
The best thing to do is to provide these users the meeting link for that meeting and provide them the Meeting ID and Password. The latter should allow them in without any issue in the even that the wrong link is being circulated.
Personally, I recommend sending a brand new invite to this meeting with updated meeting information to avoid anything like this from happening moving forward.
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I've provided the same link for a series of 7 weekly meetings. As well as emailing the links a few days in advance of each meeting, and again on the day, there is an option to use a 'magic button' on the group's website which is programmed with the same link. Most participants get in ok, a few not despite trying the various options.