I find it’s important to build a self-practice in Ashtanga Yoga, so you can take it with you, wherever you are. And you can do your own practice when you don’t have time to attend a class.
Also, it’s important to have a teacher, who can guide you to remain a student and active practitioner. It’s important to have someone you can ask when you are in doubt.
The best option is to have both, self-practice and practice with a group.
Some days the self-practice could be quite challenging and the energies of a group can motivate you again.
I believe in keeping this harmony for a sustainable practice.