A traveler is someone who can only book for themselves. An arranger is someone that can book travel for themselves as well as on behalf of other travellers.
An arranger doesn’t have the power to select their travelers - the travelers must select the people they wish to book on their behalf. To do this, go to “More Tools” at the bottom of the home page, and select “My Arrangers”.
Then select “Add a travel arranger” and enter the name of the person you wish to book travel on your behalf.
Once you have selected this person, they will receive a notification confirming that they can now book on your behalf.