You cannot volunteer. The members of the Citizen Assembly are randomly selected; they are drawn by lot.
All citizens as of the age of 16 and legally resident in the German-speaking Community can be chosen to be part of a Citizen Assembly.
The selected citizens can confirm or decline their participation. This means, participation in a Citizen Assembly is always upon voluntary basis.
For each new Citizen Assembly, the Parliament starts a new lottery.
More information on lottery is available here.