Jason, When you open a file browser to add photos, you can add multiple photos by holding down Ctrl and selecting all the photos you want. Works for me on both Windows and Chrome OS, presumably similar works on OSX and Linux as well.
If you have the Android app installed, you have a couple more options:
1. all photos from your phone are automatically synced with a private Picasa album in the cloud. In G+, go to Photos, Photos from your phone, and click-to-select multiple photos you'd like to post. Then click the green Share button at the top.
2. Within the Android app, go to Photos, and check the boxes for the photos on your phone you'd like to post. Then click the Share button at the top.
(An iOS version of the app was submitted a day or two after G+ launched but has not been approved yet. I've heard speculation that Apple will likely not allow the photo syncing capability, which would also likely delay the app making it to Apple's app store (i.e. Google will have to modify and re-submit).)