An end-to-end group messaging method or apparatus organizes a plurality of local groups of members into local-group (LG) trees and a public-group (PG) tree. Each tree has a plurality of nodes including a root node connecting to a plurality of leaf nodes. Each LG tree corresponds to a local group. Each member of a local group is associated with a leaf node of the corresponding LG tree. Each LG tree is associated with a leaf node of the PG tree. Members of a same local group may establish communication therebetween by using a group key associated with the root node of the LG tree of the local group. Members of all local groups may establish communication therebetween by using a group key associated with the root node of the PG tree.