بِــــسْـــمِ اللَّهِ الرَّحْــــمَٰــــنِ الرَّحِــــيــــمِ
That depends on the situation:

If they are many and came in straight after the prayer, and there is enough time before Tarāwīḥ to pray ‘Solaatu-l-Ishaa’, then in this case it is better for them to pray in Jamaa‘ah on their own, so as to avoid differing with those scholars who do not allow a person to offer an obligatory prayer behind (an imam) who is offering a naafil prayer.

But if they came in whilst Tarāwīḥ prayer was in progress, or the imam started to pray Tarāwīḥ shortly after ‘Solaatu-l-Ishaa’, and there is the fear that if a second Jamaa‘ah prays, the two groups will disturb one another, then in this case it is better to join the imam in Tarāwīḥ, with the intention of praying ‘Solaatu-l-Ishaa’. Then when the imam says the Taslīm after two rak‘ahs, they should stand up and complete the prayer by themselves.

Summarily, yes one can join the Imām observing Tarāwīḥ with the intention of Solaatu-l-Ishaa. This is because it was proven from Mu‘aadh رضي الله عنه that he used to pray the obligatory prayer of ‘Solaatu-l-Ishaa’ with the Prophet صلى الله عليه وسلّم then he would go back to his people and lead them in offering that prayer; for him it was naafil but for them it was obligatory.

Walloohu A'lam
Wabihi Nastā'īn