Who are the people? The Berlin Group, David Hilbert, Carl F. Gauss, Bernhard Riemann, Ernst Mach,Henri Poincaré, Hendrik Lorentz. The domain are the studies on the understanding the most basic fundamentals of physics derived from the analysis of electromagentism that most leading physicists of the time were busy with. Although Hilbert and Poincare are monsters of pure mathematics they were just doing physics by the side.
Looking at the biography of Hilbert, it starts to look like almost obvious what happened. In no way it diminishes the stature of Einstein to recognize it.
Hilbert was a monster:-) Hilbert did invariant theory, calculus of variations, commutative algebra, algebraic number theory, geometry, spectral theory of operators and its application to integral equations, mathematical physics. Einstein by the contrary, always needed help with Mathematics, since the times at University. That was how he met his first wife...
Hilbert was following the work of Einstein but this latest one was struggling to make any progress for years. In the summer 1915, Hilbert's was interested in general relativity, and he invited Einstein to Göttingen to deliver a week of lectures on the subject.
Suddenly within a few weeks, Einstein comes up with his Field equations while Hilbert follows with his foundations of physics. The fact that Hilbert fully credited Einstein as the originator of the theory, and did not engage in any public priority dispute is pretty clear was as a statement to the previous work Einstein did for years. With the collaboration and common work that happened its pretty clear, Einstein did not have the mathematical prowess and skill, to come up with those sudden breakthroughs...