If your clients have already set up a Tourbook account and created their profile or profiles then the easiest way to work with them is to ask them to invite you to join their Tourbook team.
Your client can do this easily from their Dashboard by selecting Account > Team and clicking the Invite Person button and add your details.
You will receive an invite to join the Account as a member of the team. When you accept this invite you will be automatically added to the team.
Note: Tourbook Free enables a maximum of 1 user so your client will need to upgrade to add you to their Team.
If you already have your own Account, or are already a team member of another Account, then you will be able to access the different Profiles to manage by going to the dropdown menu in the top right of the Navigation bar and going to Switch Account. Here you will see all the Accounts you are connected to and can easily switch between them.