That's a very interesting topic, as it depends on the specific people involved. When in a group, especially with someone who is there for support and not really a witch, might hinder the chances of success of that spell. When in a group with other actual witches, it's still difficult as each person has to have the same intent and goals as each other.
I myself enjoy having just myself or one other person i trust deeply to help me get the goal I am out to accomplish.
I suggest you look into it yourself, experiment. See what works best for YOU.