Entourage comes with the Microsoft news group account already set up for you. You will not find any general Mac groups on their server. You would have to create a new News account using the settings provided by your ISP for their own news server. The number of groups available, the completeness of the coverage, and the length of time the messages stay on your ISP's news server is dependent on your ISP. Some do a wonderful job, others hardly have any offerings. With Entourage you can read messages and post messages to the groups.
There is another way to access the news groups. That is through Google Groups. You can read all the messages at Google Groups, but you have to sign up with them if you want to post a message.
You can see a listing of the main Mac groups available by clicking
here. Only some of these may be available through your ISP's news server.
Matt
Mac Mini; B&W G3/300