The purpose of Extranet group work distribution is to offer a platform of collaborative work easy to administrate and to use. The management of the users is centralized in a single directory (LDAP) and is carried out in a graphic web interface (EGWM or GOSA). You therefore set for each of your users a single account, which he/she will be able to use for each of the available services (according to access rights that you would have granted). Each user created that way will get a personal storage space, an exchange space and a common storage (according to his/her access rights).
Other services are only accessible via a protected tunnel: VPN. It uses PPTP Technology (also supported: IPSEC), technology chosen for its simplicity of implementation, just some clicks under windows:
You remain of course free to implement the whole or part of these services.
To facilitate the access to your Extranet Group Work server from internet, you can use a domain name you have no matter if it is hosted at OVH or not. This will enable you to access to your server using a name such as myserver.mydomain.com and not an IP or the name by default (nsxxxx.ovh.net).
If you decide to use the mail functions, you will have to also think to point MX recording from your domain to your Extranet Group Work server.