I believe there is an argument for a separate group for experimentalist. I have found the standard physics group to be far too theoretically oriented and where easily answered trivia comes to dominate discussion. To the extent that working physicists (and engineers) such as myself find it almost useless from a professional POV.
As an example of the issue Dirk raises, I'll point to This recent question. Now, perhaps it is not the cleanest of questions, but it did come up today. It was put on hold as off-topic, with comments about how it would be best for engineering or materials. Now, given that it was tagged solid-state-physics and superconductivity (and applies to them), and that materials-science is also a reasonably used tag here on Physics SE, I think this is pretty indicative of the general feeling around here. It is pretty hard to argue with Dirk, but I also think that Physics SE should be the place for his experimental questions, as well as questions like this.
Consider this: if experimentalists are not motivated to post here, where their questions would be well received (given that they are, in fact, about experimental physics), what would make them motivated to post on a separate site which would have even less of a community at first?
Given this I think starting a new site is less productive than trying to improve participation here.
(I'm assuming that by "group" you mean a Stack Exchange site.)
Anyone can start a new SE site on Area 51. However I suspect your chances of ending up with a viable group are small. It take a lot of commitment to get a Stack Exchange site going and I doubt there is enough interest from experimentalists for the site to succeed. Still, you will never know unless you try.
Good suggestion. It's been obvious for a number of years that the culture on physics stack exchange is not representative or welcoming of ideas in condensed matter, computational or experimental physics, at all.
The theoretical questions and answers are not a problem in themselves, but for the apparent sense of entitlement I see among the established order of the 'community' who seem to want to impose their culture on everyone who seeks to ask and answer physics questions.
If the current community are not willing to share this space with others, then all we can do is set up a new one which is more inclusive of those interested in solving real world physics problems, beginners and those "layman" who should be able to have somewhere to ask conceptual questions on physics. I'd support an area 51 proposal like that.