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There is an additional factor at work here. The plural form kalosorisate is used when the addressees are plural, or singular but respected. That is to say, the plural may be simply plural, or formal when used to a singular addressee. kalosorises is singular, but used only with familiar persons; that is to say, although it is singular, it must be used in an informal and familiar way. This is exactly the distinction you see in French between the use of the pronouns tu and vous.
Kalosorisate gonis, mathites, ke alli eklekti kalesmeni ...
Explanation: The word "welcome" in Plural is kalosorisate (καλωσορίσατε)and in Singular is kalosorises (καλωσόρισες). In your sentence is Plural of course and the Greek translation of the whole sentence is: Kalosorisate gonis, mathites ke alli eklekti kalesmeni = Καλωσορίσατε γονείς,μαθητές και άλλοι εκλεκτοί καλεσμένοι.