Hi @Mark Stanfield, welcome!
If someone already has a Trello account they don't need to accept an invitation and they're invited immediately. If they don't have an account yet and are expecting an email from Trello to walk them through the invitation process, please ask them to check their SPAM folder. They can also go ahead and create a Trello account at http://www.trello.com/signup using the same email address you invited, and they'll be a member of any workspaces or boards you've already invited them to once their account is confirmed.
If they aren't receiving any emails from Trello or Atlassian and can't confirm their account, they'll need to reach out to the Support team individually for some assistance. http://www.trello.com/contact
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