Welcome Chris!
PM (private message) is a messaging system that we use on this site. If you look under the name of the person offering the chaeto, you will see an envelope. If you click the envelope, it will bring up a PM screen, just type a message to that person and when done, just send. When the other person responds, it will send you an email and you can just click the link in the email to respond.
Hope this helps.
This is a great site with tons of WONDERFUL people.