What should a worshiper do to earn the reward of praying Qiyam with the Imam until he departs, if the regular Imam prays Witr after eight rak'ahs that his friends perform, and then the friends return to complete the prayer to twenty-one rak'ahs? And is it permissible for the friends to separate from the Imam and not pray with him after the eight rak'ahs?

Whoever desires the reward of praying the whole night should complete the prayer with the regular Imam until he departs after Witr, based on the statement of Shaykh Al-Uthaymeen that two Imams in one mosque are considered as one Imam. Therefore, whoever leaves before the Witr of the regular Imam is deprived of this reward.

If your friends return to lead people in prayer after the first Imam has left, their prayer is not considered a continuation of the first prayer, but rather an independent prayer. This does not deprive whoever left with the (first) Imam of the reward of praying the whole night.

Whoever wishes to attain the reward with the Imam, and then wishes to pray afterwards, may pray Witr with the Imam and not pray Witr again, or he may add one rak'ah to the Imam's Witr to make it even, and then pray Witr at the end of the night.