Assignee details under Task Planning will display the list of consultants available in Move Cloud app’s data base. 

Currently at Move4U, user Christian is associated as VOCZ company. If you could refer below screenshot user  is created on 11 Jan.

 
 
Value code in the Flex1 is what shares detail with which company it is associated. And we can verify that by parameter EXTERNALCOMPANY

For user 'Chritian', Flex1 value is '57c908dc-3da3-432a-83a1-cfdedbbca6a3' . 

If I search this value in the code field on parameter EXTERNALCOMPANY 
- Administration>>Reference
Click on View List inform of EXTERNALCOMPANY
Enter the  Chritian's Flex1 value '57c908dc-3da3-432a-83a1-cfdedbbca6a3' in Code field and hit search.
You can see that this Code is for VOCZ. 

So, Move4u team need to update the user information from there end i.e. correct the Flex1 , and then user will start appearing under the assignee drop down list in Task Planning.


Correct value would be - 3131ca50-1e31-41f7-ab06-e545aa334a3e