You can edit your membership. To do this, go to
http://groups.yahoo.com and sign in with your Yahoo! ID and your password. If you can't remember your password, they will email it to you. If you can't remember your Yahoo! ID, post to the group and ask someone who reads the posts on the web to email it to you (Your Yahoo! ID shows up next to your post on the web).
Once you have logged in to your group, look above where it says "edit membership" (kind of small letters in light blue). Click on that. Scroll down to your options for messages. You can change to daily digest (all posts bundled into a single long email approximately every day) or -- my favorite -- special notices only. I just log in to
http://groups.yahoo.com to view the posts in my groups.