Nominations for feedback are normally issued by the line manager, although in some configurations the reviewee can do this too. You may also be able to nominate external stakeholders to give feedback depending on your account configuration, if you need help doing this please see our help page.
First, navigate to the reviewee'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 Centre.
Then click the reviewee'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 and will be able to give feedback for X.
Note: the review has to be open in order for people to provide their feedback - if you're still waiting on responses don't close it off.