I would like to know if its possible to use one login on multisites using jomsocial. The scenario is: The main site members already registered. I would like to create another site with jomsocial and register new users and the new users can use the registered contents of the main site. Is this possible?

Or new users login on the slave site and use the main site registered contents. Please let me know if this are possible as am planning to purchase this tomorrow.

Thanks.

Yes this is possible.
If you also want to benefit of the single sign-in, the "shared" user must be on the same domain.

So you can share the users accross websites but if you want that your users benefit of a single sign-in, it is required that you different websites are using the same domain (ie. xyz.domain.com)

If the users are not on the same domain, you can share the users information between websites but the users will have to re-login with the same user and password.