If the group has a public calendar with RSVPs or signups, clicking to fill out the RSVP or signup will ask for you to enter your email and add you to the group. 

If you want to join an organization that registers members through a registration form, you will want to get the URL for the form from the organization. Fill out the form. You will then get a thank you email from the organization letting you know they have your application.

Once your application is approved, any emails that go out to the whole group will also be sent to you so that you can keep up-to-date with any news or events. 

If you want to join an organization that registers members through text-to-join, you will want to get the phone number and keyword from the organization.

You will receive a confirmation text allowing you to text back to add an email to your account or set up your profile via link, which will allow you to customize even more options.

In some cases, you will be added to a group by the administrator. In this case, you will begin to get notifications immediately. Clicking any link will allow you to interact with the group.