Public Groups send universal Alerts, with these Alerts received by whomever follows your Public Group. Private Groups, on the other hand, were designed to send specific instructions to targeted recipients that you personally invite. More simply, App Users choose which Public Groups to follow while Alert Senders choose which App Users can join their Private Groups.
Premium Groups are something of a hybrid of the two. Any App User can join a Premium Group but they must pay a fee that is set by the Group Owner. (NOTE: Premium Groups are an upcoming feature and are not presently available).
Every App User has the ability to create Public Groups, Premium Groups and Private Groups. For more information on pricing, please click here.