No one was "joining the USSR" after 1945 (Königsberg). It was the Warsaw Pact of countries bordering each other with Soviet garrisons in each of them (so Soviet tanks could keep governments in check and drive in to bring them down if some wanted to leave, as in Hungary 1956, Czechoslovakia 1968, and allegedly almost Poland 1981).

Then there were "the sattelites", who could adjoin the Soviet state (like Afghanistan) or not (like Cuba). Many socialist countries were just simply "fraternal" allies without being subjegated puppet states like Afghanistan would become. Some countries fucked off from the Moscow orbit, like Yugoslavia or Somalia, and sooner or later and more or less aligned with America instead while proclaiming the Third World officially.