You can create two separate databases with the same installation.

They can then either choose the database (eg company1) from the drop down list or you can give them the URL that just goes straight to their database .. eg http://openerp.server.com:8069/?db=company1

With the above options people can see what other companies you also be looking after (from the dropdown list).

To get their individual companies pointing to their own url eg openerp.company1.com (of course preferably using https!) by getting their dns entries pointing to your server and using an apache proxy to do some redirects/rewrites to point it at the correct url. The 8069 port hides behind the http proxy.

I haven't tried to do the above http proxy bit, but I use the same process to redirect http://openerp to https://openerp.ourcompany.com [EDIT: err no just the http to https]

Here are the docs I use to do get things working with https ...

The web client doesn't do https so the webclient needs to be configured to listen only to the local host, and we'll use an apache https proxy. The original kick start I used was .. http://doc.openerp.com/v5.0/install/linux/web/index.html#configure-https

Install apache and enable the appropriate modules.

 apt-get install apache2
 a2enmod ssl proxy_http headers rewrite

For certificates you can use the local certificates. Most browsers will probably complain about them, but until you get real certificates ..

 apt-get install ssl-cert
 make-ssl-cert generate-default-snakeoil --force-overwrite

The example site config ..

#####
# /etc/apache2/sites-available/openerp-ssl
<VirtualHost *:443>

SSLEngine on
SSLCertificateFile /etc/ssl/certs/ssl-cert-snakeoil.pem
SSLCertificateKeyFile /etc/ssl/private/ssl-cert-snakeoil.key

ProxyRequests Off

<Proxy *>
Order deny,allow
Allow from all
</Proxy>

ProxyVia On

ProxyPass / http://127.0.0.1:8069/
<location / >
ProxyPassReverse /
</location>

RequestHeader set "X-Forwarded-Proto" "https"

# Fix IE problem (httpapache proxy dav error 408/409)
SetEnv proxy-nokeepalive 1

</VirtualHost>
#####

Enable the SSL site with ..

 a2ensite openerp-ssl

We can either disable the port 80 of this site or rewrite to go :443 .. I go with the redirect.

# /etc/apache2/sites-available/default
#..
RewriteEngine on
RewriteCond %{SERVER_PORT} ^80$
RewriteRule ^(.*)$ https://%{SERVER_NAME}$1 [L,R]
#..

And then restart Apache

 /etc/init.d/apache2 restart