You can opt out of messages from folks who are not in your Meetup Groups, or from anyone on Meetup who isn't the Organizer of one of your Meetup Groups.
If someone has chosen not to receive messages, the ‘Messages’ icon under their name will be greyed out. If you see that, there isn't a way to contact that member. Sorry!
It could also be that your settings don't allow people to contact you. To edit your contact settings, please go to the Email settings section of your Account page. Scroll down to Who can contact you on the site? You can adjust your settings from there.