Script Runner: Script listener- validate assignee is group member

[Michael Shechter]

Hello, 

I want to create script listener that run on issue assigned. the Script should check if the assignee is a group member based on group picker field.

if the assignee isn't a group member, the user should get a warning   message and  assignee should not change

how can I do it? 

Thanks!

[Ignacio Pulgar]

Why don't you just set the Assignable User permission to that of the group picker custom field in the project's permission scheme, removing that permission from anyone else?