You know what? A bit of an obstacle for the organizers and slightly more hassle for me (for example: I'll have to mail one of the participants so that he mails me the pictures shared on WhatsApp).
Else than that? A complete no-brainer for all involved.
I totally get why those tools are used to help organize such an event. They totally got why I refuse to come close to anything Facebook with a vengeance and were happy to work around it.
As an added bonus I was the only one in the group who shut the phone off on Sunday evening and didn't touch it again until the retreat finished the following Friday.
If people really believe that I'm an antisocial hermit, which cannot be trusted? Well, fuck them very much!