From Yuku Help

Invited

This page allows you to view all the members that you have invited in one go. The features on this page are mostly the same as the all page. You can also view these members by using the status filter on the all page.

Also on this page you can invite more members. Simply type their username (with .boardname or .u after it) into the text field.

The person will get a pm inviting them to join your board. They will not be a real member of your board until they accept your invitation (by posting).