Whenever somebody posts a request like yours on HN, I always go to the link right away, so as not to be influenced by your HN description. Then I read your description, and it is amazing how often the description on HN is different from what the website says. Yours is a perfect example of this.
Your whole description of a "Knowledge Management tool and Collaboration Platform..." is a horrible way to describe what you're doing. It's full of a bunch of buzzwords, and I assume most people wouldn't know what you're talking about, but you've done quite a good job on your website of describing the product, so stick with one message.
Another reason to go with the messaging on your website is that a 'knowledge management tool and collaboration platform' (said in your most serious voice) limits your market to professionals only, when there is no reason for that. If your massive market ends up being 25-35 year old women planning their weddings, go with it. So don't overly define your product in the early stages if you can get away with it, and in this case I think you can.
Ok, now to ACTUALLY review your website.
"Your Group Content in One Place, Organized" - that's a great start, it's simple and well defined. However, I think you could improve it a bit and make it more compelling. Your name "GemShelf" can be easily forgotten, but I think what you're trying to say is "You have a bunch of content you love and want to share. These are your Gems, GemShelf lets you organize and share your Gems".
The only reason I made the connection with Gems is because you've asked us for input. Without that, I probably wouldn't have made the connection, and therefore your name may be forgotten.
"Collect, organize and share the content that matters most to you and your group." - again, really well done. My only concern is the "group" thing, and I wonder if people will try to figure out what their "group" is, and if they have a "group". It might be better to just cut it out after "matters most to you", as you've already said 'share' which implies they'll be sharing with somebody. But maybe that just needs to continue to be tweaked. But again, great start.
"GemShelf is a single, easy to use, hub for your group’s content. Once you use GemShelf, you’ll quickly realize that there is no better way to:" - huh? 'single', as opposed to? Less is always more, so if any word here isn't needed, ditch it. I'd just say "Once you use GemShelf..."
"an Evernote Alternative, " by saying you're an evernote alternative (which you may be), I think you're casting yourself in a light that many people won't understand. Evernote is big, but not that big, and I suspect more people have heard of it than have actually used it, and used it well. This might not be necessary, again, it's putting yourself in a bucket where I don't think it is needed. You've done a good job describing yourself to this point, I never would have compared you to Evernote up to then. "Group Evernote Alternative" and then you did it again.
SCREENSHOTS!!! Fantastico!! Oh please tell me that is not what your site looks like!? You've done so well up to this point, I worry that I look at that screenshot and it looks so busy. You were going to make this simple! I know it's probably a work in progress, but go with the feature limited screenshot if you have one. It could be just that I'm looking at a minified version of the site, maybe it's fine in full detail, but it just looks busy and a bit tired with the gradients and such.
Hope this is helpful, seems like a cool product. Best of luck.