(Article assumes you are a Moderator)
If the message is brief and text only, send the drafted notice to your Site Admin via the Portal message button in the Site Admin's Workspace. Please denote the start and stop dates of the message - Your Job is done. Within 3 business days the message will appear prior-to the User's next login via your website.

If the message has images, attachments, or you need a paper-trail on whom has seen the message: Draft your message (including content) and contact your Site Admin. All current User's email addresses can be exported from your account to put into a mailchimp campaign. Not familiar with mailchimp? We can handle all that for you.*
*Depending on your Portal Account package, additional fees may apply per incident.

The messaging system within the Portal itself is segregated per Workspace, and composing a message in each and every Workspace is likely NOT a good fit.