If you want to have a collaborative gallery, then the easiest way to do it might be to create a Flickr account to store the gallery's images, create a Juicebox gallery to display the images form this Flickr account and give all the potential image providers the login details for the Flickr account (so that they can upload images and add titles and captions via the Flickr interface). Obviously, this does not allow casual visitors to your web site to upload images to your gallery but allowing this might pose a security risk (see my notes below).

If you did not want to use Flickr, then you could use perhaps purchase Showkase and use it in its Gallery Manager theme to manage a gallery online via a web browser interface. Again, you'd need to give people access to the Showkase admin section. People could then add images (and titles and captions) to existing galleries via the Showkase interface (which you can check out by logging into the Live Demo Admin).

Alternatively, you would have to somehow provide a web interface so that:
(1) Users could upload images.
(2) User could add titles and captions.
(3) Have the gallery's XML file updated accordingly.

Unfortunately, this is not available in Juicebox-Pro and, although technically possible to achieve manually (perhaps using a third-party library for image uploading, for example), it would be a complex and time-consuming task to provide such functionality and knowledge of a server-side scripting language (such as PHP) would likely be required.
Also, you would probably need some kind of security measure. Flickr and Showkase require login details but if you just provided some kind of upload module on a web page, it might be open to abuse and visitors to your web site would be able to upload whatever images they like to your server and gallery.

You could perhaps circumvent #3 above by having Juicebox display images from a designated folder on your server so that the gallery's XML data is always generated dynamically when the gallery is displayed (and a static XML file does not have to be updated each time images are added). Please see this forum post for details.

You could also have image titles and captions embedded in the images (as EXIF or IPTC data) so that they do not need to typed in separately.
The PHP script that lists the images in the designated folder could also extract the image titles and captions. (Again, in-depth knowledge of PHP would be required.)
If you wanted to allow anyone to upload images, then it is perhaps unlikely that they would already have titles and captions embedded in their images and you might still want to provide a way for them to be able to enter them manually.

Either way, it would be a huge task to undertake.

Unfortunately, I'm not aware of any ready-made package that might make this any easier to implement. I'd just need to try a web search and see what comes up.

I don't know if my Flickr suggestion above is perhaps an option (I'm not sure if you want to allow only a certain group of people to be able to add images to a gallery or if you want to extend this ability to anyone who visits your web site) but it's certainly the easiest to implement.