How to set Data Validity Range based on a selection?

Hi all
I have a table of events (1 row per event) containing a list of participants (1 participant per column).
Now on another sheet I would like to create a drop down (Data Validity Range) populated only with the participants of a selected event.

Example (Event Table)
1: Event A; 12.12.2017; Berlin; Michael; Freddy; Robby
2: Event B; 11.11.2017; Paris; Phil; George; Bono; David

Detail Sheet
Selector: Event B
Dropdown containing names only from Row 2 (Event B)

By using INDEX I was able to get one value (i.e. D2=Phil). But I can’t figure out how to make INDEX return the whole range D2:G2

How can I get a range with INDEX? Or what is the “correct” way to accomplish this selective dropdown?

If you want several values on one row in the dropdown, you concatenate one INDEX() or any other expression by column, but all will be on the dropdown cell.