Assignment groups hidden for all groups expect a particular group

[phr]

Hi All,

I have a requirement and it says :

Suppose there is 

1) Group A

2) Group B

3) Group C, D, E...and so on

Whenever the user group Group B logs in and opens the incident form in the Assignment group Field Group B member should be able to see Group A.

For Groups C, D, E...and so on ...Group A should be hidden.

(No one should be able to assign incidents to Group A except Group B)

How to achieve this?

[Mark Manders]

Again: what business rule? How needs a business rule here? What do you expect a business rule to do in the context of this query?

[sahana1998]

@Mark Manders 

tried above script include not working