Two or more sorcerers together can combine their powers and cast a powerful spell. Sharing the Mana requirements among all of the participants to prevent any calamity from otherwise happening.
In order to cast a spell as a group a ritual must be devised and properly prepared. A spell cast in a ritual generally has a longer casting time than normal. And thus the mana rating will be lowered slightly because of it. The spell must be known by at least one of the participants in order to be successful.
The group still has a Mana Threshold rating and a Mana Limit. If these values are exceeded then a calamity may occur that will be the same for all of the mages involved in the casting.
Group Mana Threshold
Use the mage with the highest Mana Threshold as the base and add 10 for each additional member.
Group Mana Limit
Again use the mage with the highest Mana Limit as the base and add 3 for each additional member.
After the casting is completed, the mana of the spell is evenly divided among all of the mages that took part in the ritual. (round up). This counts against their individual Mana Threshold for the day. So even group casting cannot allow mages to cast spells indefinitely.