Users management in XO takes an important place. A recurrent question is "why managing users in XO while XCP can do it?". Let's analyze how XO is dealing with users and why it's not the same goal as XCP.
It's more a reminder, but because of its location, XO is a "hub" on top of all your pools/servers. That's why it's easy to centralize users management, you can give them rights globally: pools or stand-alone servers are no more isolated. XO make the "glue". For example, when using XCP, you need to configure every pool with its backend. If you have a lot of servers or different pools, it can be tedious. With XO, you have one place to configure user rights, plus you have one place to configure your auth backend.
One place, one backend
That's why XO will be easier: as a central place, you just need to configure your auth service once. We will provide fine grained permission for this summer, so we think that kind of feature + OpenLDAP backend can be really useful in your existing infrastructure.
Are you trying to recreate CloudStack?
Oh god, no! :D As we always think, there is a space between vanilla Xen and "big" cloud stack, like OpenStack or CloudStack. XCP is really great with its Xapi, the only thing missing is a light, open source and powerful interface for that. And we always think "modular": we DON'T want to create a big monolithic software for cloud management. First, we'll focus on core features, and then we extend it with plugins, and they'll never be mandatory for playing with your virtualized infrastructure!