I would like to ensure only certain group people can assign tickets certain group.

[Rohith16]

Hi all, I have a requirement to assign a particular assignment group in Incident, change , and SCTask only by certain group members. Every one should not be able to assign this group...

For example say there are 3 groups Group A , Group B and Group C only the members of Group A and Group B should be able to select Group C assignment group in Incident, change and sctask form.

[Dubz]

You could use isMemberOf() in an advanced reference qualifier on the assignment group field eg:

if(gs.getUser().isMemberOf('<group name>')){
return '<sys id of group C>';
}

You would obviously need to build this into a proper structure to return all other groups as well but you can use the isMemberOf() method to include or exclude group C based on group membership.

[Rohith16]

Should I enter the script in Advanced reference qualifier as it is?or should create a script include and call it in advanced qualifier

[Dubz]

You'll need to create a script include because there will be other conditions required. If you just add the script as above then the only assignment group available in that field will be group C and it will only be available to members of group A or B. I was just giving an example of how isMemberOf() works, it's up to you to now deploy it.