Click on the 'more' icon (top right) to add photos or videos to albums of your friends. The owner of the album will be notified and decides if they will be added.
Don't want visitors to be able to contribute to your album? Turn this off in your album settings (three dots top right » uncheck 'enable contributions') or for all your albums in on your profile page under 'settings'.
If avatars are enabled by the owner of the album all the avatars of contributors are shown below the album.