What's your criteria for this set of users?

If it's just all users that register between now and a set point in the future, you could use the setting under Admin/System/Registration - Give new users this role. Very simple.

If you're wanting to distinguish between some users based on another criteria it will not be easy and may not be possible at all.

However, if you have an SSO you could have a lot more flexibility - you can assign roles based on rules in your CRM that are passed to Lithium when the user signs in to ensure the appropriate entitlements/status. An example might be whether the community member is a paid-up subscriber or not.