Hi! Maybe this is a misunderstanding due to FOSDEM lingo - call for participation is not an registration/invite, it is the equivalent of call for papers (that is, talk submissions).
FOSDEM [1] is open to all (there is no registration or fees) - and several thousands of people come every year.
We don't do poster sessions, but we have a dedicated slot for lightning talks in one of the two days. Shoot us an email at info@qosf.org and we'll see if it's a good fit.
FOSDEM [1] is open to all (there is no registration or fees) - and several thousands of people come every year.
[1] https://fosdem.org/2019/