We provide proof-theoretic results about deliberative STIT logic. First we present STIT logic for individual agents without time, where the problem of satisfiability has recently been shown to be NEXPTIME-complete in the general case. Then we study STIT logic for groups of agents. We prove that satisfiability of STIT formulas involving groups of agents is undecidable by reducing the problem of satisfiability of a formula of the product logic S5n to group STIT satisfiability problem. We also prove that group STIT is not finitely axiomatizable.