As we know while sending a mail to a distribution group, we can view the members of the distribution group by clicking on the ‘+‘ symbol next to the email address in outlook.
In case you want to disable expanding the distribution group, you can achieve it through ADSI Edit as discussed below:
Launch ADSI Edit, find your distribution group and select Properties.
Select Attribute Editor and find out the attribute ‘hideDLMembership‘. Now set the value to ‘True‘ to hide the membership of the DL.
Reference : http://exchangeblog.pl/en/tag/distribution-group-en/