Be sociable! Engage with other people! It could be almost anything - the Masons, the Green Party, the Boy/Girl Scouts, the Rosicrucians, the pickleball league, the FBI, the circus, the Justice League of America. (Just not the Republican Party.) Get off your duff, put down that game console and engage in collective action.
This particular sign invites people to become members of the Saint Louis Art Museum.