Lawyers are invited into a matter via an email invitation.
Property owners can add a lawyer into a transaction via the New Sale or New Lease form.
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.
Matters won't show up as Properties, Sales or Leases in your profile as you don't own the transaction, but if you click the indices up the top Properties, Sales and Leases you will see the transactions you're invited to.
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.