I want to make a blogportal where friends can sign up and create a own blog?

Yes, b2evo can do that.

Is it possible to make the sign up process automated?

Yes but "automated" is a very vague description of what you might be thinking in your head.

Is it possible for user to register a subdomain or url, or will it be the registered username?

Yes users can each use a different subdomain or domain (but those domains need to be registered with a registrar)

Is there possible to make restrictions on number of photos or total storage to use on each user?

There is an upload quota per user in the works, it will be available in a future release.

Is it possible to make a frontpage that shows randomly posts from theusers blog - randomly, based on popularity and so on?

Yes.

Is there any newsletter plugin that the users can implement on their blog?

There is a built-in newsletter feature which may or may not suit your needs. You'll have to be more precise about what a newsletter means to you. Again a thousand different ways to think about it and they can all be called "newsletter"

With newsletter I mean that user can set up a e-mailregistration form (i.e. just e-mail adresse), and get a list of visitors he can send newsletter/mail to.

b2evo can do newsletters like that, but if I understand correctly, in your case you would need a different newsletter for each blog. Correct? This will be supported in a future version.

Regards subdomain. I meant that if users can use the site domain with a subdomain, and as today must admin create this subdomain? Or is it possible in set up make it possible for user to choose and register subdomain on site.

Yes subdomains are easy. Users will be able to choose their subdomain. The admin will only need to configure that the server responds to *.yourdomain.com. ( * meaning "catch all subdomains" here)

Automated sign up. What I meant can user fill out a username and password form and register themselves?

Yes they can.

Or must it go through admin?

No they don't need to if you don't want them to.

Is it also possible to registration and set it up like that only admins can regoster new users?

Yes, if you want to. All this is configurable in the User settings.