noun which may take the genitive case particle 'no', noun
open to all; free-for-all
Describes an event or activity that anyone can join without restrictions.
This event is open to everyone.
The training is optional, so feel free to join if you're interested.
任意 is more formal and often used in official contexts to mean 'voluntary' or 'at one's discretion', while 自由参加 is specifically about participation being open or optional.
Compound of 自由 (freedom) + 参加 (participation).