public Collection<JID> getAll() {
        Set<JID> everybody = new HashSet<>(administrators);
        everybody.addAll(members);
        return Collections.unmodifiableSet(everybody);
    }
Returns a read-only, unmodifiable collection of everyone in the group.

@return a read-only Collection of the group administrators + members.
xmppserver/src/main/java/org/jivesoftware/openfire/group/Group.java