First, navigate to the appraisee's profile page. As a manager you'd first go to Your Team. If you're an Administrator, you could do this through the Administration Center.
Then click the appraisee's name and click the feedback tab:
On the right, use the search box to find the name of the person you'd like to nominate.
When you've found their name, click Send an invitation to give feedback for X.
The nominee will be sent an email immediately and will be able to give feedback for X. The email looks like this:
There is no limit to the number of people who can be nominated to give feedback.