Do we understand correctly that you have a Web Interface action when executing which you want users to be able to select only groups that are members of specific Business Unit? If so, unfortunately, there is no such possibility. As a solution, you can try using filtering options (e.g. LDAP filter) in the action settings to specify what groups should be allowed for selection. If you have issues with the configuration, please, specify the action in question and how groups to be displayed can be determined.
Also, please, specify what version of Adaxes you are using. Is it 2009.1?