I will start by saying that I do not have a particular opinion on this topic. This post is more to get you thinking so that you will discover if perhaps you have an opinion on this topic.
I am strictly a solo practitioner. This is because it is what I am accustomed to, what is easiest for me, and what I find comfortable. The possible side effects of assisted magick (or group magick) do not factor into this decision. However, it has been taught that the magick of multiple people is stronger than the magick of one person. The same tends to be taught about prayer in the Christian culture. In the case of prayer, I have participated in obliging prayer requests many times. This in itself is a form of magick in my opinion, but not for my personal workings. Prayer energy goes to God, not directly to another person. So, the reason to ask others for help in raising magickal energy is obvious: The more energy, the stronger the energy. There are witches/Wiccans/pagans that love being involved in group workings. There are “solo practitioners” who may ask long-distance allies to send them energy. (I put that in quotations because I am not sure you can truly be a solo practitioner if you are asking others to help you. Perhaps it is possible if these instances are few and far between.)
On the other hand, I have also heard of the potential bad that can come from assisted magick. I have heard that asking someone to send their energy to you or working in a group links your energy to the energy of another person (or group of people). If these are all good, stable people you might not be bothered by that idea. However, people are not always what they seem. Imagine you have a chat friend named Sally that lives 300 hundred miles away. You have been talking with her for 2 months and she seems awesome. You ask her to send you some of her energy for a spell you are working on to increase your chances that the spell will work. Sally agrees and sends you the energy. The spell works. Great. However, what you don’t know is that Sally suffers from Bipolar Disorder. Because you didn’t sever the energy bond after your spell work, Sally can unintentionally draw out your energy when she is having an episode. This leaves you feeling drained and not understanding what is wrong with you. There is also the possibility that someone in a circle you have formed has a hidden agenda and is purposely drawing on your energy without asking your permission. Now of course these things can be prevented by having a protection protocol in place. However, as an analogy, abstinence is safer than safe sex. Sometimes condoms fail. I have also heard a witch state a very simple reason for not wanting the energies of others involved in her magickal workings: she wants to know that her spells work because of what she has done and not what another has offered.
I repeat that I have no particular opinion on this subject, so I ask that nobody gets offended and tries to argue with me. I am simply presenting the arguments I have heard. If anybody has an opinion on this topic, especially one gained from experience, I am interested to read it. Blessed be, y’all.