You have lots of options. You can use JPAIdentityStore for authenticating users from db(has a standart way of doing it).
If it doesn't fit your needs you can write your own IdentityStore with implementing org.jboss.seam.security.management.IdentityStore Interface. There is a lot of samples you can find around. Try to search the forum. The main point you can find samples is jboss.org but is down for a little!!! maintenance.( nearly about 2 days:( )