GroupMe is a free app that allows you to message multiple people at once. I know what you're thinking--you can already text message multiple people at once, all you have to do is add multiple numbers to the "To:" field in your texting app--even non-smartphones have this ability. GroupMe is different, though: It doesn't just let you send messages to multiple people, it lets you have a group conversation.
Let me explain. When you text multiple people at once, all you're really doing is sending the same message to a bunch of different people. However, if those people text you back, then their response only goes to you--there is no "Reply All" button in text messages.
GroupMe is like a default Reply All button. GroupMe sets up a special number for each texting conversation, so when you text multiple people at once they get a text message from that number. Then, when they reply to the text message, their reply is sent to everyone in your GroupMe conversation. So it's like a group chatting app, except via text message.
Your friends don't have to have GroupMe installed for it to work--which is a good thing, because that means that people who don't have smartphones can also partake in the fun. Only the conversation starter needs to have GroupMe installed. However, if you want to directly message people, the recipient of the message will need to have the app installed on their phone.
GroupMe isn't just about texting, though--it also has conference calling, a mapping service (which gives the approximate locations of the other chat participants), and a (still-in-beta) "Ask a Question" feature.
GroupMe's main screen looks like a texting app--it's got all of your grouped conversations, a search button, a prominent "Start a Group" button, as well as buttons for Home, Contacts, Featured, and News. The Featured button takes you to a list of GroupMe partners (such as the TV show Dexter and the artist Ke$ha) and lets you start groups based around said partners. The News tab is just sort of a random newsfeed.
Starting a group is as easy as pressing the big green plus button at the bottom of the home screen and adding a couple of worthy contacts. Message them once and your contacts receive a couple of texts--including a note that they've been added to a group and a list who else is in the group with them. Any member of the group can start a conference call by dialing the special phone number your group has been assigned for the duration of your group conversation.
The original conversation starter can also disband the group--but once a group is disbanded, its special private group number ceases to exist.
GroupMe is an innovative way to get multiple people involved in one conversation. It can be annoying to have your phone constantly go off whenever someone replies to the group, but it's great if you're trying to get a group of people together to meet up, or if you have a bunch of friends who really love to talk to each other.