To use categories to create a dynamic distribution list, switch to the By Category view then select the category group you want to send a message to. You can use this method to create meeting requests, appointments, task, or journal items. In Outlook 2007 and 2010 (and above), to create an appointment linked to a contact or group, choose New Meeting Request then cancel the invitation to convert it to an appointment.
In Outlook 2010 and up, select the category group you want to send to then click the E-mail button in the Communicate section of the Home ribbon. This will open a new email message with the contacts in that category selected.
In Outlook 2007, select the category and look on the Actions, Create menu for the New message to command.
In Outlook 2003 (and older versions), Look on the Actions menu for the New message to... command.
If you select a single contact, New message to is on the right click Context menu (under Create in Outlook 2007 and Outlook 2010).