I recently read the JoS article about advanced thoughtforms.
The first point says:

The elemental should be given a form in relation to the goal. You can shape this out of the ball.

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What shapes can be used? What shape works best for certain purposes instead of other ones?
I am planning to make an advanced thoughtform to be used by me and a few friends (yep, all SS) as we share a common goal. Can all of us feed the thoughtform? Or only the creator might?
Last question, is it even ok to make one using grey energy (it will have to bring woe to an enemy)?
I would be happy to hear your own experiences and advices with that kind of magic too

Don't make stuff like group thoughtforms. It's way too easily exploited and does nothing to advance the people involved. It's a flawed practice in and of itself.
If you want to curse an enemy, then just do a group working for it instead. Start it together on a suitable date and keep it going with intensity for 3 to 9 days, or 40 if it's a bigger endeavor.

If you use grey energy in the working, then always keep it purely external, and clean yourself thoroughly afterwards.

Also to answer your question, thoughtforms made from grey energy, if at all feasible, would require one to be incredibly advanced in order to sustain successfully without it going out of control and bringing ruin to you. Stay away from these kinds of dangerous things. There's a good reason why that kind of stuff is not on the JoS, just like death mantras. It's too dangerous and there is lacking knowledge on how they can safely be used, if that's even possible at all.

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To my opinion creating the Thoughtform in animal or sub-human form is better as the Thoughtform will have a basic intelligence, so it is easier to deal with a similar shape. Anyway this is personal choice.

What you want to create is a group thoughtform, so you want to create an Egregore. In this case you can control the Egregore altogether, so you should bind it to a Sigil that is used by any member of the congregation to command it, plus a name that you only know that will be included in the Sigil.
Many SS together mean high Energies involved, you need to be sure any SS is commanding and feeding the Egregore with the very same intention of others, otherwise confusion may arise.

Grey energy can be dangerous for backfiring. You can command the Egregore to pick up and deliver grey Energy to a target, but you will need to clean the Egregore after, or it will become "dirty" and possibly negatively influenced.

Thank you guys for replying me. I'd like to clarify my question as I have been a little vague lol, also giving feedback.

Shael said:

(...) then just do a group working for it instead. (...)

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Yep, I know regular workings, I have done (either with positive and negative scope) many over the years, but wanted to know more about thoughtforms. I might not explained very good while asking that I do not want to create a group thoughtform for black magic purposes, but instead I am thinking about one that will act for negative purposes to a specific person. And a second one, to be used in a group, for bringing positive things to the group members. About the second one, Every member of the group would regulary feed it energy

Shael said:

Also to answer your question, thoughtforms made from grey energy, if at all feasible, would require one to be incredibly advanced in order to sustain successfully without it going out of control and bringing ruin to you. Stay away from these kinds of dangerous things. There's a good reason why that kind of stuff is not on the JoS, just like death mantras. It's too dangerous and there is lacking knowledge on how they can safely be used, if that's even possible at all.

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It's not the first time I find myself in the need to perform black magic, unfortunely. I think I have by now enough exprerience in either meditation and magic usage to handle this. BTW there are on JoS website articles that teach about black magic. Also runes can be used for black magic purposes (Hagalaz, Thurisaz, Isa, Nauthiz for example) without problems if one susuffices in knowledge. I like your suggestions about keeping always your aura clean and keep the negative energy strictly external, these are very important to anyone new that is willing to learn black magic in a safe manner.

Gerecht Ror said:

What you want to create is a group thoughtform, so you want to create an Egregore.

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I did not know about Egregores, but by a quick internet lookup I have learned that they are pretty similar to "standard" thoughtforms, expect they are operated by a group of people instead of just one individual. Can you please suggest me a good source to learn more about Egregores?