You can subscribe to alt.aromatherapy, alt.folklore.aromatherapy, or own.health.aromatherapy, the three Usenet newsgroups devoted to the use of essential oils for healing. Alt.aromatherapy has the most traffic, by far, and is the oldest. Sometimes flame wars erupt, sometimes good information is exchanged, and sometimes flaky, even dangerous, advice is given.

The idma list is an email-based communication tool for all those interested in aromatherapy. There are about 1200 members, composed of suppliers, practitioners, consumers and interested folks.  It is unmoderated, and flame wars do erupt. Some people have been kicked off, but that is rare. Commercial spam not allowed, but many suppliers have extensive signature lines that are advertisements. Good source for networking, finding information, exchanging information, and starting 'trends'.
To join, send mail to aromatherapy-subscribe@idma.com

Yahoo.com has numerous aromatherapy lists. Two of the most popular are:

http://finance.groups.yahoo.com/group/oils_herbs_etc/
http://health.groups.yahoo.com/group/ATFE/