From the desktop site:
On your Meetup Group home page, you'll find each upcoming Meetup listed in the center of the page along with an RSVP button.
Click on the RSVP button or the name of the Meetup event to view the details of the Meetup. From here, you can RSVP by clicking Yes or No under the Are you going? header on the right side of the page.
If the Meetup allows guests to attend, a prompt will appear asking how many guests you're bringing, or if it's just you. You may also be prompted to answer RSVP questions set up by the organizer. When you're done, click Finish RSVP to complete the process. You'll now see yourself added to the attendance list on the right side of the screen.
When you receive an email announcement of a new Meetup, you can click on the Yes or No button in the email to register your RSVP directly from your email inbox.
The email will also display some other upcoming Meetups for the group, so that you can RSVP to those as well.
If your plans change, click the Change RSVP button to change the number of guests you'll be bringing, or to alert your organizer that you are no longer able to go.
From the mobile apps:
To RSVP to a Meetup event, first navigate to the Meetup group whose event you’d like to attend. Tap the event on the Meetup’s calendar, and then select 'RSVP.'
Follow the prompts to complete your RSVP to the Meetup.