I am looking for open source software to use as a basis for a new website and not sure which one will be best to use for this purpose.

The website is basically a muti-user guestbook (community) where each user will have
- its own very simple self-administrated page
- with a few lines about himself, optional photo and
- a list of messages that other users have left for him
- links to leave messages to other users

Each registered user can post a message to another user. The user who received the message can then allow this message to be public or private (just for himself) or delete it all together. So each personal page would be a list of meesages from other users.

Would be nice to have features such as
- message notification by email when someone left you a message.
- allow an existing user to invite a new user by sending an email invitation from his page.
When a new user clicks on the email string, he would go through simple registration process and create his personal page with the message already waiting.

I would apprecaite any comments/advice as what open source software could best fit for this purpose.

I do not mind customizing HTML, CSS, simple PHP modules, etc. Not sure if the closest to what I am trying to do is blog, or wiki, or something else. Of course if there is some sort of multi-user guestbook open source sw, then this is what I need.

thanks
Greg