It’s simply a fact that most witches in the world are solitary — and there are numerous reasons for this. However, even solitary witches can participate in group spells, and do so from time to time. So are group spells more potent than spells that are cast in a solitary setting? The following article will attempt to address group magic versus solitary witchcraft.

Power in numbers
If a product is five time as powerful as an original formula, then is it more effective? That’s a common sense question that comes with a common sense answer: Yes. This same reasoning can be applied to casting spells. If you have five witches (or less, or more) working toward the same goal, then you’re going to have that much more energy working in unison. However, a group spell is only as powerful as the group casting it.
The power of one
Some people have more focus than others, and as stated above: A group spell is only as powerful as the group casting it. If you have a group of five or so witches and only one of them possesses the focus to effectively move a spell, then did the group spell even work? In this case, the witch in question would have been better off casting a solitary spell. 
Expectations
Whether or not group magic beats solitary witchcraft depends mostly on your expectations. If you’re wanting to attract a lover, do you really need multiple people casting the same spell alongside you? (Not saying it wouldn’t work, just saying it’s a bit unnecessary!) If you’re wanting to target a group of people, on the other hand, you’re going to need a group of people. For example, witches across the world have banded together to hex Donald Trump. A single witch likely couldn’t affect any of the energies that surround the most powerful man in the United States. But a well organized crowd of several hundred witches could do more damage.
All-in-all, it’s really up to your judgment on whether or not group magic is better than solitary. I’m of the personal opinion that both are equally valid and important.