Facebook just added another feature to Messenger as part of its quest to have their service become an all-encompassing communications app. This feature lets you make free group calls using Messenger's VoIP service, and you can chat with up to 50 people at once.
You'll need the latest Messenger update for Android/iOS to call groups.
You make calls the same way you do with a single person. Just open the group you want and tap the phone icon in the top-right corner. On the next screen, you'll be able to select which members of the group you'd like to call. Your recipients should receive Messenger calls after you press "Start Call."
Even if someone isn't available at the start of the call, they can still hop in later if the call is still going. Also, if you're waiting for someone specific, you can continue to send notifications to them about the call in progress.
You can't make group video calls yet, but you can probably expect that feature sometime in the near future so that Facebook can compete with other group messaging services such as Google Hangouts and Skype.
One last feature that isn't yet available on group calls is the ability to mute, neither yourself nor others on the call. This would be especially handy with a large group, as it would be very easy to have the conversation overtaken by ambient noise, and people accidentally (or not) talking over one another.
Facebook seems pretty intent on making your phone number obsolete. Who knows, maybe in a year or two your alma mater will resort to constantly pestering you for donations through Messenger instead.