Both regular or e-mail user do not need to take any special action to send a message to a Cerebro. It is sent out automatically, immediately after publication of the message in the forum - just like in many other one-way e-mail notifications in most modern software.
In other words, the message published in the Forum immediately gets available for both types of users. Availability of the message for a certain user depends on his/her role on the task (assignee/follower/client) and access rights to the task Forum.