Sometimes I want to share photos my friend right after after we take the shots. I want those photos to appear in my Google+ Instant Upload album immediately, rather than waiting for it automatically do it.
How can I force "instant upload" to run?
From the Google+ app, select
Settings -> Instant Upload and press the
Upload all item, it will just upload any photos that haven't already been uploaded.
As noted in the comments, you can also
share individual photos in your album via Google+.
Make sure that your connection settings permit you the backup - you must be using a WiFi connection if under Back up Photos you have chosen Wifi only and you must be charging if you've put at tick next to While charging only.
The same procedure could be done through the Google+ app instead of the Photos app.
Settings/Accounts & Sync/Google/(choose Google Account if necessary)
Google Plus Photos is probably already checked. Un-check it and re-check it. This will force it to re-sync immediately.
Someone should develop a button to do all this automatically. I edit all my photos in Google Plus before posting on Etsy.com. I force the sync so I can work continuously.
HTC One S running Android 4.1.1