He joined the socialist movement around 1898, becoming a member of the Social Democratic Party of Galicia (GPSD), an affiliate of the Social Democratic Workers' Party of Austria. The GPSD, led by Ignacy Daszyński, was formally Marxist, but dominated by Polish nationalists close to the Polish Socialist Party (PPS). When the Ukrainian Social Democratic Party in Galicia (USPD) was formed in 1899, the GSPD became the Polish Social Democratic Party (PPSD) and the Polish nationalist current was strengthened. Grossman led the resistance of orthodox Marxists to this current. Along with Karl Radek, he was active in the socialist student movement, particularly in "Ruch" (Movement), which included members of the PPSD as well as of the two socialist parties in the Kingdom of Poland, the PPS and the Social Democracy of the Kingdom of Poland and Lithuania (SDKPiL – led by Rosa Luxemburg and Leo Jogiches). He was the main figure in the newspaper "Zjednoczenie" (Unification), which took a line close to the SDKPiL, against the pro-PPS politics of "Ruch"s main organ, "Promień" for which he was censured by the PPSD and its newspaper "Naprzód".