Most of the emails you receive through Meetup will include the reply-to address of the person who sent you the message. But some emails that are sent through Meetup come 'from' firstname.lastname@example.org. That email address is a no-reply to address, and it's used for outgoing email like new Meetup announcements and reminders, notifications about new greetings or message board posts, or messages for you from other Meetup users.
Since many different kinds of emails use that same 'from' address, how you reply to the person sending the message will depend on what sort of message you're trying to respond to.
If you're having trouble figuring out how to respond to the sender of a message you received, here are some things to look for:
Most emails that come from a Meetup member or Organizer will include a link back to the sender's profile. It'll look something like this:
Click the link to the sender's profile, and you should be able to 'send email' back to the person who contacted you:
If there's not a link to the sender's profile, then look for a link in the email you received to view or respond to the message
You'll see these sorts of links in emails you receive about things like greetings or comments being posted on a Meetup event page.