Creating a user

This will show you how to create a new user using the UserService in Umbraco.
If you want to create a new user, you'd use ASP.NET identity APIs like it is used in core.

Assigning the user to a user group

Permissions aren't administered for the specific user, but rather for the user group(s) that the user is a part of. So to add our new user to a user group, we first need to get a reference to the user via the GetUserGroupByAlias method, and then use the AddGroup method for adding the group to our user:
// Get a reference to the default "Administrators" user group
UserGroup adminUserGroup = (UserGroup) us.GetUserGroupByAlias("admin");
// Add the user to the user group
To make sure that these changed are saved to the database, we must also make sure to call the Save method. The GetUserGroupByAlias method takes the alias of a user group - eg. admin for the default Administrators user group.