You can add other members to your organisation by going to Settings > Users.
To send an invite, click Invite new user, and then type the person's email address. They will then receive an email with instructions on how to join CHASER (if necessary) and how to access your organisation in CHASER. For security reasons all existing members of the organisation will also be notified of the invitation.
Invited users do not require access to your organisation’s accounting system. If they do not then they'll still be able to use all Chaser's features, but links from Chaser to the accounting system (in order to edit contact details or view invoice details) will not be functional.