I have set up an Odoo 8 server with three databases.  I have attempted to invite new users to one of the databases, but they get a 404 when they try to follow the link in the invitation email.  The problem appears to be that Odoo is relying on a cookie or something local to the browser to know what database to access.  I figured out a workaround after examining the server access logs excerpted below.  Notice that the second attempt worked, and all the others failed.  Every access attempt corresonding to a "INFO None werkzeug" log entry failed, while the one that generated "INFO ART werkzeug" succeeded.  ART is the name of the database we're after, as you can see from the db=ART node of the URL.

2014-10-17 16:51:05,067 720 INFO None werkzeug: 127.0.0.1 - - [17/Oct/2014 16:51:05] "GET /web/reset_password?token=7ddLQc45Pmt6awpj2Pvj&db=ART HTTP/1.1" 404 -

2014-10-17 16:52:14,655 720 INFO ART werkzeug: 127.0.0.1 - - [17/Oct/2014 16:52:14] "GET /web/reset_password?token=7ddLQc45Pmt6awpj2Pvj&db=ART HTTP/1.1" 200 -
2014-10-17 16:53:31,292 720 INFO None werkzeug: 127.0.0.1 - - [17/Oct/2014 16:53:31] "GET /web/reset_password?token=7ddLQc45Pmt6awpj2Pvj&db=ART HTTP/1.1" 404 -
2014-10-17 16:55:52,536 720 INFO None werkzeug: 127.0.0.1 - - [17/Oct/2014 16:55:52] "GET /web/reset_password?token=7ddLQc45Pmt6awpj2Pvj&db=ART HTTP/1.1" 404 -
2014-10-17 16:56:16,001 720 INFO None werkzeug: 127.0.0.1 - - [17/Oct/2014 16:56:16] "GET /web/reset_password?token=7ddLQc45Pmt6awpj2Pvj&db=ART HTTP/1.1" 404 -

To work around the problem, I instructed the users that before attempting to follow the link in the invitation email, they should first visit the equivalent of http://odooserver.mycompany.com/web/database/selector and select ART (or whatever appropriate database).  Then, without trying to log in, return to their email invitation and follow the invitation link.

This is obviously a bug that occurrs when there are multiple databases (perhaps only when inviting someone to a database that was not created first?).