Cobot allows you to send emails to either a single member, a group of members or all members.
For newsletters we advise to use a dedicated newsletter service like http://mailchimp.com - sending newsletters without using a dedicated service might get you into trouble with email spam. In addition newsletter services allow you to track open rates, send HTML emails and such.
To send an email to a single member on Cobot go to that member and click on their email address displayed below their billing address.
To send an email to all members go to the member list page and click the Send Email to List button at the top of the list.
To send an email to a group of members go to the member list page, use the filter field at the top of the list to shrink the list to the people you want to send an email to and then click the Send Email to List button.