Working with paintbrushes, cameras and craft kids, a networked group of artists open the summer show at Sloan Fine Art tomorrow night. “+2” invited a couple artists from past exhibits to invite two more: “someone they know or not, someone they owe a favor, someone they’d like to see get a break, someone they’ve always wanted an excuse to meet.” The showing of small collectible works opens from 4 to 6 PM tomorrow and is on view through July 18 at 128 Rivington Street.