Many Papertrail users need to be active on more than one account and accepting invites for subsequent accounts is really simple:
I have received my invite for the Papertrail - Office account and need to click on 'ACCEPT INVITATION' to complete the process:
Because your email address is already registered with Papertrail, this link will take you to your 'INVITATIONS' page where you need to add your password before clicking on 'LOG IN' to complete the process fully.
Logging in takes you to your 'ACCOUNTS' page where you will now see all the accounts you have been invited to and accepted the invite from. Simply click on the one you need to work on and you will be taken to your dashboard page.