WordPress MU

So I want my users to be able to login to different MU sites with same username. Possible?

Yep.
You have to add each user to the blog you want them to log in to.
Login to the blog in question as an administrator. Go to the Users tab, and fill in the form at the bottom "add user from the community).

Alternatively click on Site Admin, the blogs, then click edit next to the blog you want to add a user to and scroll down. On the bottom right hand of the page you will see an Add user form. Fill it in and update the settings.

I'm not sure if there is a way to bulk add users though.

Thanks cafespain, thanks a lot... hmmm... StayPress... I can smell it:)

Hm... Actually I was wondering if You can have the same users on different MU installations...

Scenario:

So, imagine that you're already a user at MUtravel.com and by default you are able to use your user name at MUsports.com, MUmusic.com,...

That doesn't mean that you have blog(s) created on other MU sites (MUsports.com, MUmusic.com,...). Just a user name. Of course you would be able to create blog(s) if you want to.

Hmm, that would be an interesting one....
I think, theoretically, using my Multidb code I could set two installations up to use the same users table - which may accomplish it....

Other than that, I'm not sure how easy it would be

If it's done off one install, why not? :) The sub-blogs could be separate sites and still be mu-ified.

Pssst.... What do you think the wp_sites table is for?

2, 3, 4 "main sites", and all the subblogs you care to offer.