Lawyers are invited into a matter via a direct email.
Property owners can add a lawyer into a transaction either when creating it, or alternatively in the access tab of a transaction.
When you're invited into a transaction as a lawyer you'll receive an email notifying you that you've been added to a file.
It will also include a link to the particular transaction on Vendorable.
If you haven't signed up before you'll have to sign up and create your profile before the link in the email you received will work.