ugh i was typing this!
Make a trigger, put in the on Codtion field.
Player distance (lost guys name) < 3
that means when the player is less then 3 meters from the lost guy the trigger will "pop", now in the ONoactiveon field put
[(lost guys name)] join player
make sure you put the guys name with [] on both sides. now to make this a little more real you might put after the join thing.
(lost guys name) sidechat "I am sure glad to see you sir!"
etc.
hope this helps.