You can set your group so that private replies are enabled via a button so named at the bottom righthand corner of a blank reply form at your group's online site. That button toggles the 'rpivate reply' option on or off.
Another option would be to enable your group's online directory. This allow participating members who have opted-in to show their name in that Directory to email one another.
Failng the above, your member(s) may share email addresses with one another and then take their discussion off-list.
OK — HTH