When a facilitator invites you, VSTA sends an email called "You're invited to" followed by the client's name. It says which role you have been given.
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Follow the link in the email. It opens a page called "You have been invited to VSTA".

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Choose Accept and sign in.
VSTA adds you to the client and signs you in, so you do not need a sign-in email this time. Next time, sign in from the sign-in page with the same email address. Sign in explains how.
The invite link lasts 14 days. If it has run out, the page says "This invite has expired. Ask your facilitator to send a new one."
If you were already a member of the client, accepting signs you in and leaves your role as it was.