In short, both these methods of adding contacts require both parties to actively add each other in the same time period. So even if you post your briar contact code publicly, only the contacts you are adding as well will be able to connect with you.
Thanks. I meant more with respect to users spamming forums - to send a message to a particular forum do I just have to know of its existence? Or is it that each user only sees messages from its contacts on each forum?
A Briar user can either create a new forum, or get invited via a Briar contact that is already in an existing forum. There's some more info on this topic in Briar's user manual if you're interested: https://briarproject.org/manual/
Briar also has the ability to introduce contacts to each other, so while it doesn't support totally public forums, you can spread information between contacts fairly effectively.