i've been twice--once for a week, once for three months. both times i went solo. for a woman going solo, modest dress is a must--in fact, this is a must for any woman going to morocco, more for cultural respect reasons than anything else. i was occasionally followed in the streets. i made some great friends (mostly men, actually), i got into a relationship. i was hollared at (as were the european men i met while there) in the medinas and approached in cafes. however, not for one moment did i feel like i was in any kind of danger when i was by myself--it was just a hassle sometimes. if a woman is dressed modestly and is being harassed in the streets, it is absolutely acceptable for her to draw attention to the man and shame him. i would recommend any bus traveling either be done in djellaba or with a man, or both.
in general, i really never had any problems and meeting up with people to accompany me on shorter travels or for dinner was easy--that traveller's bond is far-reaching