I created this stored Proc, hope it works as expected...
Create TRIGGER [dbo].[register_yafUser]
ON [dbo].[yaf_user]
AFTER INSERT
AS
Declare @UserID int;
Select @UserID = [UserID] from Inserted;
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
insert into yaf_UserGroup (UserID, GroupID) values (@UserID, 3);
-- Insert statements for trigger here
END