It currently isn't possible to post to a Usenet group via email; however, you can use email to post to a Google Group as long as you're subscribed to it. You can figure out whether a group is a Usenet group or a Google Group by clicking the About this group link on the group's homepage. Usenet groups have names with words separated by periods, such as 'rec.food.cooking.' If you'd like to post to one of these groups, you'll just need to visit the group's homepage and click the + new post link or the Reply link.
Please note that you can still receive and read Usenet group messages via email.