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 to view the details of the Meetup. Then you can RSVP by checking Yes or No and clicking the RSVP button. Use the fields provided to add guests or a comment to your RSVP, which will be added to the Meetup's conversation stream.
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 to.
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.
***Please note: if you’re not already a member of the Meetup Group hosting the Meetup event, you’ll be prompted to join the Meetup first. Your RSVP will be complete once the Organizer of that group approves your membership.