Compulsory

[Zipporah Schumaker]

Obligatory; enjoined by authority; necessary; due to
compulsion.

[Kit]

Required by law or a rule; obligatory.

[Ernesto]

[Optional].

For when you are [organizing] a [compulsory] event or meeting and people choose not to attend because for no other reason, they just dont want to.

[Jessica]

is [sth] that is [required] [for everyone].