You can add as many users to Chat Flow as you need.
To add a new user, click on the profile icon in the top right of your screen and select 'User Management'.
Click on the '+ New User' button.
From here you can enter the user's details and select the role (and associated permissions) you want to assign to the user.
For more details on adding users, see the relevant section in the Clickatell Portal User Guide.