Ezequiel,
It is not that strict. In some cases SIGs and special tasks can be created so I would not be completely strict about Members willing to help in the TSG (Technical Steering Committee) in WP1 who allocate a reasonable PY. SITAC is big enough and help is needed also to backup persons who has management tasks.
On the other hand WP leaders, task leader and national coordinators must allocate WP1 PY…
Rgds,
Ilan