GroupMe offers super-easy group texting and conference calls from any phone
Setup for GroupMe is ridiculously easy: just put in your mobile phone number, and your group will be created and get its own number. You can text commands to the new number to add friends or change the name of the group, or you can invite your friends to add themselves by texting a code to the group number. If a group gets too spammy, you can text #mute to the group number to stop getting messages for a while, and #unmute to start again.
Advantages of GroupMe: even the most old-and-busted dumbphone on the market can use it, as long as it supports texting, because GroupMe works like a regular phone number. Even without texting, you can still call a group number to start a conference call. Disadvantages: it's US-only for the moment, so international users are out of luck.