In this paper, we study conditions which ensure the existence of backward flow invariant domains for semigroups of holomorphic selfmappings of a simply connected domain D. More precisely, the problem is the following. Given a one-parameter semigroup S on D, find a simply connected subset Ω ⊂ D such that each element of S is an automorphism of Ω, in other words, such that S forms a one-parameter group on Ω.On the way to solving this problem, we prove an angle distortion theorem for starlike and spirallike functions with respect to interior and boundary points.