He is praying as an imam and he wants to delay Witr

It is mustahabb for the last prayer that a Muslim prays at night to be Witr, because the Prophet (peace and blessings of Allah be upon him) said: “Make the last of your prayer at night Witr.” Narrated by al-Bukhaari, 998; Muslim, 751. 

This command from the Prophet (peace and blessings of Allah be upon him) is in the sense of it being mustahabb and preferable, not in the sense of it being obligatory and compulsory, because it was proven in Saheeh Muslim (738) from ‘Aa’ishah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) used to pray two rak’ahs after Witr whilst sitting down. 

Al-Nawawi (may Allah have mercy on him) said:  

The correct view is that he (peace and blessings of Allah be upon him) performed these two rak’ahs after Witr sitting down, to demonstrate that it is permissible to pray after Witr, and to demonstrate that it is permissible to offer naafil prayers sitting down. He did not do that all the time, rather he did it once or twice or a few times. 

The well known regular Sunnah prayers mentioned in al-Saheehayn and elsewhere in reports from ‘Aa’ishah and from a number of the Sahaabah in al-Saheehayn state clearly that the last of the Prophet’s prayer at night was Witr. In al-Saheehayn there are many well-known ahaadeeth which enjoin making the last of one’s prayer at night Witr, such as “Make the last of your prayer at night Witr” and “Night prayers are two by two, and when you feel that dawn is approaching, pray one rak’ah”, etc. So how can anyone think, after reading these ahaadeeth and others, that the Prophet (peace and blessings of Allah be upon him) regularly prayed two rak’ahs after Witr and made that the end of his night prayers? Rather what it means, as we have stated above, is that it is permissible to do so. This is the correct answer.  

Shaykh Ibn Baaz also said, explaining the reason why the Prophet (peace and blessings of Allah be upon him) prayed two rak’ahs after Witr: 

The reason for that – and Allah knows best – is to show to the people that it is permissible to pray after Witr. Fataawa Islamiyyah, 1/339 

So if you want to pray Tahajjud at night, it is permissible for you to pray Witr with the congregation, then to pray after that as many rak’ahs as you want two by two, and do not repeat Witr. 

You also have the option of not praying Witr with the congregation, and delaying Witr so that it will be the last of your prayers at night. 

In this matter you have to pay attention to the congregation praying with you; if there is no one else but you to lead them in praying Witr, and if your not leading them in Witr will lead to them not praying Witr or not praying it properly, then you should pray Witr with them. 

Shaykh Ibn Baaz was asked: 

If I pray Witr at the beginning of the night then I get up at the end of the night, how should I pray? 

He replied: 

If you pray Witr at the beginning of the night then Allah enables you to pray qiyaam at the end of the night, then pray whatever Allah enables you to pray two by two, without Witr, because the Prophet (peace and blessings of Allah be upon him) said, “There cannot be two Witrs in one night.” 

And it was narrated from ‘Aa’ishah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) used to pray two rak’ahs after Witr sitting down. Fataawa Islamiyyah, 1/339 

With regard to your saying that you will pray one rak’ah with them, intending that as a naafil prayer and not Witr, this is not an action that is prescribed in sharee’ah, because the Prophet (peace and blessings of Allah be upon him) said: “Night prayers are to be offered two by two.” Narrated by al-Bukhaari, 472; Muslim, 749. See al-Mughni, 2/539. 

Al-Haafiz said: 

It was understood from this that naafil prayers should not be less than two rak’ahs, except Witr. 

And Allah knows best.