Synchronise the trigger to the waypoint at which you wish the group to wait. There is a synchro button at the top of hte mission editor, just click it and drag and drop.
You should aslo learn about switch triggers, which are a particular and very important variation on this theme. Tute in the Ed Depot.
Note that a unit cannot have waypoints. Waypoints may be enjoyed only by groups. Obviously a group may consist of only one unit, but it is important to get these details right. Forum questions and indeed entire missions have come to grief on misundertandings of apparently trivial details.
Edit: hmmmm, 2 mins and 5 seconds..... Mud_Spike was right, I
am getting old.