Hey Yannick!
I just wrote a short summary from Thessaloniki. In terms of participation it was much below the expectations.
On one hand, it’s a constant struggle of event organisers. We always have to figure out was to get people - I think that adding a nice, social event (we had this amazing Brasilian social kitchen going on during EuroAlter Capus a few weeks ago - with fantastic food prepared from waste, but also singing, dancing and bits of performance) - which usually saves money and makes it a bit more attractive than many hours of discussion in a closed space.
I think you need to try with those who confirmed - ask each one of them to bring along someone with relevant work. People know each other, at least a bit;)
For some of the participants it might be useful to send a reminder, or basically repeat the whole operation of posting and sending emails, a few days before - many of us cannot plan a week ahead;)
I will think of other ways to do it. We will get a good amount in Brussels and Berlin, I am sure of it.