So lets say someone posts in area chat they have a T3 purple secret map. Do i need to have the same secret map to do that secret dungeon with them, or does only 1 person in a party need to have a map in order for everyone to do that secret dungeon.
Every secret map run ive done with people I say hey I have more want to do them, and they all leave.
U need or the 3 other people the same Map and same Tier von a Run.
The reason for this is, not to get scammed again.
Best update so far. 
okay so my understanding is that all 3 party members need to have the same secret map… thank you for clarifying
I personally run chaos gates throughout the week and do maps in a big bunch with guild mates
Yup, so say you run SV.
Either a Rare, Epic or Legendary map will drop (or relic but that is different).
After the Chaos Gate is done you group with 3 people who have the same tier/rarity map as you.
Then everyone joins the same channel, 1 person goes to their map door and they check the button that all 4 people run the map at the same time, using up the map for everyone but getting 4x the drops.
Typically if you have multiple maps you’ll just shout in area chat for a new party, since the random 3 other people you found don’t also have a second map of the same tier/rarity to run twice.
@Redrowen is more accurate. Same tier. So if you got a epic/purple Rift, do that one over the Plateau. It is the same result but a smaller dungeon.
Note that if you have more than one secret map that would be valid, you can change the selected in the pop up that asks you to confirm usage of said secret map. Typically I just have the one I want gone higher up in my inventory.