Is it permissible to perform the Witr prayer in congregation after twenty rak'ahs of Tarawih, then those who have completed Witr depart, while the Imam completes the remaining rak'ahs with those who remained of the worshipers? And will those who departed be rewarded with the full reward of Qiyam al-Layl?

The reward for performing the night vigil (qiyam al-layl) is established for whoever prays Tarawih with the Imam until he finishes all of the prayer, even if the Imam prays twenty-three rak'ahs, as stated in the hadith of the Messenger of Allah, peace and blessings be upon him: "Whoever prays with the Imam until he finishes, Allah will record for him the reward of standing for an entire night." If one prays in a mosque where there are two Imams leading Tarawih, it is advisable to remain until the second Imam finishes to complete the reward, as the two Imams are considered as one. So, whoever performs witr with the first Imam and then wishes to pray Tahajjud with the second Imam, should perform one rak'ah after the first witr to make it an even number.