Hey Wess! Invite links to Infisical orgs expire after 3 days, so if it's been a while that's almost certainly it. Can you confirm roughly when it was sent? Also a couple of quick things that'd help me figure out what's going on:
1. Can you paste me the full link (you can DM it to me if you'd rather)? Sometimes Outlook/Gmail mangle long URLs or wrap them in tracking redirects that strip the params. If that's happened, opening the email source and copying the raw link directly into your browser usually fixes it.
2. What email are you trying to sign up / log in with? It needs to be the exact address the invite was sent to.
3. Just to sanity-check, the link should start with
<https://app.infisical.com/>
(or
<http://eu.infisical.com|eu.infisical.com>
/ your self-hosted URL). If it points somewhere different from where you'd normally log in, that's the problem.