Firstly: When to Pray for Rain
It is mustahabb to pray for rain at the time of the Eid prayer. This is the position of the majority: the Malikis, Shafi`is, and Hanbalis.
Secondly: Gathering to Pray for Rain and Supplicating
It is legislated to gather to pray for rain, supplicate, and implore Allah the Glorified. Consensus on this was related by Ibn `Abd al-Barr, Ibn Battal, Ibn Rushd, and al-Nawawi.
Thirdly: Permission of the Leader When Praying for Rain
It is not necessary to seek the permission of the leader in order to gather to pray for rain.  Al-Mardawi says: “The difference of opinion regarding requiring the leader’s permission is when the pray in congregation. If they pray individually, there is no argument that his permission is not required.” (Al-Insaf, 2/322) This is the position of the Shafi`i and Hanbali schools, Ibn Baz, and Ibn `Uthaymin.
Fourthly: Disbelievers and Dhimmis Gathering for the Prayer for Rain
If dhimmis gather for the prayer for rain, they are not prevented, but they are ordered to stand apart from the Muslims. This is the position of the majority: the Malikis,  According to them, they gather at the place of prayer at the same time as the congregation but are isolated to one side. They do not gather before or after the congregation, for it is feared that if they seek rain before or after them that rainfall might coincide with their prayers, which would be a trial of faith for the congregation. Shafi`is,  Al-Nawawi says: “We have mentioned that the position of our school is that they are prevented from gathering while free-mixing with Muslims. They are not prohibited from gathering while remaining distinct. This is the position of al-Zuhri, Ibn al-Mubarak, and Abu Hanifah.” (Al-Majmu`, 5/72) and Hanbalis.  Ibn Qudamah says: “The gathering of dhimmis is not advisable. They are the enemies of Allah who have disbelieved in Him and exchanged the bounty of Allah with ingratitude. They are far-removed from having their prayers accepted. If the Muslims are given rain, they might say it is due to their supplications and the acceptance of their prayers. If they gather, they are not prevented for they are seeking their provisions from their Lord, so they are not denied that. It is also not far-fetched that Allah would grant them, for He has guaranteed their provisions in this world just as He has guaranteed the provisions of Muslims. They are ordered to stay separated from Muslims because there is no guarantee that a punishment might not befall them and befall those with them.” (Al-Mughni, 2/328)
Fifthly: Taking Livestock to the Prayer for Rain
It is makruh to take livestock to the place of the prayer for rain. This is the position of the Maliki school and a group of Shafi`is. It is one position among Hanbalis. It is the position of choice of Ibn Qudamah.
Sixthly: Where to Pray for Rain
It is mustahabb to pray for rain in the desert, like the Eid prayer. This is the position of the majority: the Malikis, Shafi`is, and Hanbalis.