As part of its OAuth authentication process with other apps, Meetup doesn't provide email addresses of its users. (I refer to this official Meetup forum question, and to this page in the Meetup API docs—search the page for 'email'.)
Twitter doesn't provide email addresses either. However, Meetup seems nicer than Twitter.
People use multiple Twitter accounts (I know some who do). But people don't use multiple Meetup accounts (at least supposedly not).
When registering new users through this difficult class of OAuth authentication providers (those which don't supply an email address) one might ask each new user directly for some email address, or might not. Requesting this is normally recommended.
If an app uses Meetup authentication (and it doesn't request and confirm an email address during user registration), and uses another form of authentication also (even added later) then there's no way to identify the same user, if or when they sign on by a different way.
So Meetup authentication (without email) is only good if the app is forever limited to using Meetup authentication alone. With that permanent limitation, in such an app, nobody (mysteriously) will run into the problem of having more than one account.
Of course, having Meetup as the single method of authentication is useful, reasonably, only to apps which are already limited to Meetup users.
Keeping the UI simple (by not requesting an email address when people register) means the app might never have email addresses. But that might be okay if an app uses Meetup authentication alone, forever.
Then one need not bother people with asking for their email address when they first use an app. The ease of that emotional UX moment when new customers are forming their first impression of an app (and making their initial commitment to it), from the standpoint of building a customer base—depending on the app—could be considered more important than ever knowing their email addresses.
BTW, omniauth-meetup is a good gem for doing Meetup authentication in Rails.
Copyright (c) 2013 Mark D. Blackwell.