Can I reference posts in my emails or on my web site/blog ...?

Yes! but but please ensure you give the proper credit:

1. Credit the Author, and if they have a website reference it.
2. Credit YJGG with link to YJGG and the original post.

In terms of netiquette, it is generally considered better to reproduce part of the post and post a link to the original rather than reproduce the whole post. If the post is not your own please ask on YJGG for permission from the author. This will ensure you are complying with Yahoo's Terms Of Service (TOS) agreement.

Refer To A Specific Post
If you want to reference a post that you or others have made in your emails or on a website simply reference the message id. Here's some examples:

Jimmy Bruno on Re: Technique question for Jimmy Bruno
http://groups.yahoo.com/group/jazz_guitar/message/3545

Dan Adler on Re: Rest stroke picking
http://groups.yahoo.com/group/jazz_guitar/message/14791

Sid Jacobs on Re: Waltz For Debbie
http://groups.yahoo.com/group/jazz_guitar/message/20076

Refer To All Your Posts
You can also use the advanced search message archive by user to list all your posts automatically. For example, here are YJGG member Dave Woods posts

Refer To The Group Posts
You can even syndicate the whole groups posts as news items using RSS. A number of members are using JavaScript RSS box viewer code or the open source program zFeeder on their website so they can show the group posts. For example: Catfish Club News

See Also: What's an RSS Feed ...?