The Red Army was not particularly brutal in its invasion. That is to say, the invasion did not deviate from established Stalinist norms: you're not seeing a great deal of variance compared to, say, the Soviet occupation of Lithuania shortly thereafter. It wasn't pleasant, but it was no Red Army invasion of Germany; the fighting was brief. The most pain comes after the invasion, with the Soviet annexation of the eastern Polish territories: the Kresy. At that point there were the usual purges, mass arrests, expulsions and whatnot (and of course, the mass executions at Katyn). Since Stalin wanted to fold these lands into Byelorussia and the Ukraine, the purges were of a wider scale than normal, as ethnic Poles in general were targeted (in the quest to make the lands solidly Byelorussian or Ukrainian in character). Many Poles were arrested and sent to labour camps; these prisoners would form the backbone of the Anders (pro-Western) army and the Polish People's Army (pro-Soviet).

As for how anyone in Poland could align themselves with the Soviets, very few did: between Poland's historical rivalry with Russia and the very recent Russo-Polish War, there was little love for the Soviet Union. Communism, after the failure of it to spread much beyond the borders of Russia, became seen as a Russian artifact, and so was also despised. For those that embraced leftism, you could also choose Polish socialist parties, or even one that embraced full communism but followed its own line rather than that of Moscow's. As such, those Poles that went full Moscow-line communism were true believers, convinced that communism as practiced by the USSR was the best path to improving the lot of Poland and the Polish people, and believed so until their dying days (see "Them": Stalin's Polish Puppets for extensive interviews with some of the leading figures at the end of their lives). Not even the Stalinist purges of 1938 that saw most communist Poles murdered by the NKVD had much effect on the faith of the remainder. Most notably, these Poles often thought nationalism was a poison dividing the workers of the world, and so appeals to "Poland" as a separate nation state in competition with the Soviet Union would have no power. Quite the opposite: this would have been a typical bourgeois argument in their eyes. Some of them, such as Wanda Wasilewska, argued for Poland becoming another constituent people's republic of the USSR, rather than continuing to exist as a separate state (even communist controlled). All of them supported the Soviet annexations: this was Polish communist policy back to the 1920s.