Is there a simple way of doing this trough the Admin portal, or some other way?
I've around 3.5K members, and want to start reaching out to them via email, offers, News, even menwrs that haven't posted for say 3 months or so.
Any help will be appreciated, thanks.
How about the newsletter functionality built in to SMF?
https://wiki.simplemachines.org/smf/SMF2.0:News_and_newsletters#Newsletters
try to find a mod...
if you know a bit about databases you could extract the information from the members table in phpmyadmin or similar.