Yes, you can give discounts by customizing your members' plans. The best time to do it for new members is before you confirm them, because when confirming the first invoice is generated.
To customize a plan go to a member and click on the Customize Plan button in the drop-down of the Plan box. In the following form you can change all parameters of the plan for just that member.
The changes will be effective immediately. You can customize a plan any time later as well.