All praise is due to Allah, and may peace and blessings be upon the Messenger of Allah.
The basic principle is that the person suspected of having caused the evil eye should be asked to perform ablution for the afflicted person, if it is known or strongly believed that they are the one responsible. This is based on what happened with the Prophet ﷺ when Amir ibn Rabi'ah afflicted Sahl ibn Hunayf with the evil eye. The Prophet ﷺ instructed him to perform ablution and pour the water over Sahl.
However, if the person responsible is unknown, some scholars have held that the prescribed treatment is Islamic ruqyah, and that washing should only be requested from someone known to have caused the evil eye.
If it is strongly suspected that the person responsible is one of a group, or singling out a particular individual might cause harm or embarrassment, there is no harm in asking everyone to perform ablution without accusing anyone in particular.
It is permissible to collect the ablution water of several people in one container, and it is also permissible to use each person's water separately. There is no requirement to keep the water separate or follow a particular order when washing. The purpose is to ensure that the water used by the person who caused the evil eye reaches the afflicted individual.
Shaykh Ibn Baz (may Allah have mercy on him) was asked about requesting a group of female students to blow into water for treating someone afflicted by the evil eye. He permitted this and explained that reciting Quran, blowing into water, and supplicating to Allah for healing are all permissible, provided that no one is forced to participate.
The request should be made gently, without accusing anyone in particular or hurting other people's feelings.
And Allah knows best