To add collaborators, visit one of your albums, click 'Share' and make sure that the option 'Let people I share with contribute photos' is enabled.
'Contributors will need to sign in to their Google Account to add photos or videos. Their content will be attributed to them, and they can make edits such as adding captions to, rotating, or even deleting the pictures that they've uploaded. Of course as the owner of the album, you have complete control over who can contribute content. You also have the ability to edit this content as if it were your own. You can manage collaborative access for contributors on the 'Shared with' list by just toggling the 'plus' icon Add Contributors next to their name. A green icon means that user can contribute,' explains Google.
'A limitation worth noting: contributors won't be able to upload to collaborative albums from the Picasa software,' mentions the Google Photos blog.