This is easy to do once you set a Meetup setting.
On meetup.com > Settings > Email Updates, scroll down to "iCal Attachments" and check the box to "Get email reminders with iCal attachments."
Now each Meetup email announcement and confirmation will have a
meetup.ics calendar-event attachment. Desktop Gmail and Android Gmail (I don't know about iPhone) will handle the .ics file nicely.
Desktop Gmail will display Events in this message at the top of the message with an Add to Google Calendar button.
In Android Gmail, tap on the
meetup.ics attachment to open it in Calendar. It'll show all the meeting info with an ADD TO CALENDAR button at the bottom.
This should work for email from any sender as long as there's a
.ics attachment describing the event.
Note: In Google Calendar, Settings > General has a setting "Events from Gmail: [x] Add automatically". Experiment with this. It does not seem to blindly add all events in Gmail messages.