What do you mean by blog portal?

Blog Portal is like this site: http://www.blog23.com/
A site that can host other bloggers

I think that b2evolution have this option: the multi-user option. but i need to check if it is possible to create a main page with a register option.

There is a good commercial option called: http://www.netartmedia.net/blogsystem/

But i prefer the open source, if i'll get the options i need: host other bloggers.

Yes a blog portal will be possible, but to get it to a professional standard it will take some serious work to do.

I suggest you start by reading this item on [url=http://www.fralenuvol.com/albero/servizi/webmaster/scripts/b2evolution_autoblog_plugin.php]the Autoblog plugin[/url].
Next read the [url=http://forums.b2evolution.net/viewtopic.php?t=8146]topic on the same plugin[/url] on this forum.

If you like what you see, you can think of implementing B2e into your site.

Good luck