A circle requires a minimum of three casters to form. . in order to participate in a circle, a caster must be able to cast spells of the first level or higher.
A circle is a means of raising the level of a spell. to cast a spell in a circle, a caster contributes a single spell slot to the circle. Every three spell slots is equal to a slot of one level higher.
Ever circle must have a leader, the leader is considered the caster of the spell and must know the spell being cast.
In order to cast a spell of a level higher than his highest level spell slot, he must make a concentration check of (10+level of spell*2-level of spell slot contributed by leader) if this check is failed all spell slots consumed by the circle are wasted and the leader must make another concentration check of the same level or take damage equal to 2d10 times the level of the spell of a type that the DM thinks makes sense for the spell or if the spell is a single target spell with a negative affect the dm could choose to inflict the spell on the leader. Any necromancy spell should deal necrotic, any healing spell radiant, fireball would do fire damage though if you fell generous as a DM you could heal them instead, I recommend force damage for spells like shield, Antimagic spells like counter spell, antimagic field or dispel magic prevent you from casting magic for a number of rounds equal to the level of the spell. The DM should inform the players of any possible consequences for failed circle magic
You must assemble a circle before you can start casting the spell or contributing spell slots
Time to assemble a circle
3-10 members (their movement and action on each of their turns)
You also need to assemble this many people in the first place meaning you have to convince them to participate and get them all in once place, possibly feed them
10-400( 1 round per member)
400-600(1 minute per member)
600-1000(2 minutes per member
more than 1000 (36 hours plus an aditional day for every 100 people beyond 1000)
A professional army of mages may form much faster, my marching band can form 500 people up in ten seconds. complete newbies may take an hour for 10 people to form a circle.
with X being the number of levels above the spell slot contributed, y is equal to the number of spell slots of the same level required
y=3^x
so the same level 3^0=1,
one level higher 3^1=3
2 levels: 3^2=9
3 levels 3^3=27 (at this point you will need to include NPC's and summons in your circle)
4 levels 3^4=81
5 levels 3^5=243 (this is a major plot event)
6 levels 3^6=729... (probably not going to happen)
7 levels 3^7-2187
You can also use combinations of leveled spell slots. So you could have one mage contributing a level 2 spell slot, and 6 contributing level ones to get a level three spell, if you have higher than level 9 spells in your campaign you can use them with this. I personally recomend making wish a level 10 spell if you choose to use these rules
Special rules
Wizards can write down higher level spells than they could normally learn,
clerics and druids can use any spell from their spell lists.
I know this is extremely complicated, if people like the system I may create a google sheets to do the math for you