I could do email, but would rather do private messaging. I think it is more visible and would result in more reads than a notice. Also looking for internal, non-emailed newsletters and what-not, stuff that isn't really fit for a notice. Thanks for the responses! So I am guessing there is no mass conversation addon in existence yet?
A notice will be far more visible than a PM, especially if you style the notice nicely. It'll be the first thing that hits them when they visit the site, well before checking for PMs (assuming they bother to do so).