Receiving join board requests from invited members - why?

[Maria Watson]

I'm using free trello and sometimes (not always) when I send an invite link to new members I receive a request that they want to join the board and they have to wait until I authorise it. Other times people can automatically join using the link.   I thought it might be when the invited person wasn't already using Trello but I tested that and it's not correct.  Can anyone shed some light? 

[Cody Curry]

Hi Maria! Generally we see this when the direct link to the board is shared instead of the board's invitation link.

The share link is available using this method: https://support.atlassian.com/trello/docs/inviting-people-by-using-a-shareable-link/

The direct board link is what you find in your browser's address bar while you're viewing the board. When someone opens a direct board link of a board they're not a member of, they'll have an option to request to join:

My gut says that some people might be finding the direct board link instead of the invitation link!