Add group system

Initial commit, not tested in multiplayer yet. Some revisions still left
to do.

SQF/dayz_code/groups/selectPlayer.sqf

@@ -0,0 +1,13 @@
+private ["_inviteButton","_playerList","_target","_uid"];
+
+disableSerialization;
+_playerList = _this;
+_inviteButton = findDisplay 80000 displayCtrl 3;
+_uid = _playerList lbData (lbCurSel _playerList);
+_target = _uid call dayz_getPlayer;
+
+if (player == leader group player && {!isNull _target} && {count (units group _target) == 1}) then {
+	_inviteButton ctrlShow true;
+} else {
+	_inviteButton ctrlShow false;
+};