It was . . . . . . . . honour for Rohan to be . . . . . . . . invited guest on the Golden Jubilee Function of the school.

As 'honour' is not specific, we can't use 'the' (Because 'the' is definite article).
'H' in 'honour' is silent and we pronounce the word as - 'Oner'. Hence, we will use 'an' before the vowel sound.
'Invited' is an adjective which describes 'guest'. As the article has to be used before 'invited guest' and it is a singular noun, hence we will use 'an' ('invited' has a vowel sound, so 'an' is used).