Practically overflowing with radiant portraits, “Mimi Gross: Among Friends, 1958–63” at Eric Firestone Loft should be of the major crowd-pleasers of the moment, but it feels like it’s flying under the radar. Channeling a rare, quicksilver sense for detail, Gross was only in her late teens and early 20s when she made these pieces in crayon, paint, and pastel, working in New York, Provincetown, and Europe, where she and friends traveled northern Italy by horse-drawn carriage, doing shadow-puppet shows in small villages. (To think that young artists today believe a night at Berghain is bohemia!) Two highlights of many: Grand Street Boys and Grand Street Girls (both 1963), whose many young sitters look interesting enough to sustain a few seasons of prestige television about the Lower East Side during the Kennedy years. The show depicts an artistic life that was just getting started, and already being lived very well.