Greeting a mixed crowd
Usamah bin Zaid (radi Allahu anhu) reported: The Prophet (sal Allahu alaihi wa sallam) passed by a mixed company of people which included Muslims, polytheists and Jews, and he gave them the greeting (i.e., saying As-Salamu `Alaikum)
[Sahih Al-Bukhari and Muslim]
We benefit from the above Hadith that it is permissible to initiate the greeting when addressed to a group Composed of both Muslims and non Muslims, or one Muslim and a number of non Muslims, but he should intend the greeting to be directed towards the Muslim(s) among them, because as the above Hadith proves that the Prophet (sal Allahu alaihi wa sallam) greeted an assembly that included a mixture of Muslims and non Muslims.
And Allah knows best!