The 55 Mercer Gallery begun in December 1969, is one of the loose associations of artists that has now become a familiar variation on the traditional gallery structure.   It is an artist-run organization whose members share no common aesthetic philosophy or style of work.   They share the rent of the space and do with it what they want. There are one-person and group shows, events staged around a theme, film showings, and guest speakers--all without any strong need to form a cohesive group. 55 Mercer is a place, its members are individuals, and all that can be described is what is going on at any given moment.