From the desktop site:
Joining a Meetup Group is pretty simple. When you come across a Meetup Group you'd like to join, click the Join us! button on the right side of the navigation bar.
You'll be taken to a page that will ask you to fill in your Group profile for that Meetup Group. You may need to add a photo or answer a few questions, depending on how that particular group is set up.
Many Meetup Groups are open to anyone, though some require Organizer approval before new members can join. If that's the case, once you've submitted your profile, you'll see this message letting you know.
You'll receive an automatic email when your membership in that Meetup Group has been approved.
From our mobile apps:
Select a Meetup Group that interests you and then tap 'Join Us' on the homepage.
Follow the prompts to submit your membership request to the Organizer. You’ll be notified once your membership is approved.
Please know that if you haven't already registered a Meetup account, you'll need to create one before you can join any Meetup Groups.
Also, if you have an account but you aren't logged in, you'll need to log in before you can actually join the Meetup Group.