Hi Alex, 

The Assigned To picklist is populated by your list of active users. To see this, you will need to go to Start > Configuration > Users & Security > Users. From here, you can remove the licence, and disable any users who are no longer with your company that you would like to remove from the list. Please let me know how you get on with this. 

Kind Regards, 
Workbooks Support

Hi Sam,

i worked this through with support from one of your colleagues via email and I will share the learning.

The assigned to pick list was showing lots of Ques and your colleague explained the following:

The Picklist Values displayed below are related to Queues that have been created for that particular Record. Queues are a concept in Workbooks which enable you to assign records to a specific group or function and allow you to notify the relevant people involved as as previously mentioned are Record Type specific.

To Delete the Queues on Opportunities, you will have to go to the Opportunities Landing Page and select Manage Queues. This will display all the Queues currently associated with the Opportunities Record, you are then able to select the Queue you would like to delete which will remove it from the Assigned To Picklist.

So i went in to each record type and manage the old ques, these users were never in the new Workbooks system. It was to do with user identity when we migrated from an old dynamics CRM to Workbooks. It was quite a simple fix in the end managing and deleting old or irrelevant ques.

thanks
Alex