answer:
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.

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 attend.