This rule will be useful to award points to users when they register an account at your site.

Event: User - User account has been created
Action: User points - Grant points to a user

In the Action settings, make sure the Arguments configuration for User: is configured the way you choose. For example, as admins can create user accounts, if you would like the registered user to still receive the welcome points then the User: argument for the Action needs to be set to "registered user".

Additionally, you can add conditions to check the role of the user so that you can give different # of points or category for the points depending on the role of the user.

Event: User - User account has been created
Condition: User - User has role(s)
Action: User points - Grant points to a user