You right click on the person you want to level (in your group) and select Grant a Level. So if you have 20 levels to gift you have to do it 20 times.

It also causes a pop up on the recipients screen (so they can choose to accept or refuse your level up). So you have to accept each level too.

It doesn't give an indication of how many levels you have to gift (only an issue if you've granted a few levels to different characters). it becomes greyed out when you have no levels left to gift.