4 letters:
goop, grew, grip, grua, roop, rupe

5 letters:
croup, droop, drupe, grape, grewe, griep, gripe, gripp, groep, groom, groos, groot, grope, gropp, grube, gruen, grupo, grupp, roope, roupe, troop, troup, grupe

6 letters:
groome, groove, groups, gruppo, throop, troupe, groupe

7 letters:
group's, grouped, grouper, groupie, groups'