I really like
GroupMe. It's free and it allows you to create an unlimited number of groups where everyone in the group can text everyone else (you can also text individuals, if you just need to send a quick message to someone). Others are not required to have the app (they will get a text message to their mobile number instead and can reply and follow the conversation using their phone's SMS app), but it exists for the major mobile platforms (and you can use the Web site to send/view messages). It also has a great feature that gives every group created a phone number for conference calls. Calling the number will call everyone and join them into the same voice call, allowing you to quickly talk to a group of people without dialing them all individually.