Here is the entire verse again:
καὶ ὡς ἀτενίζοντες ἦσαν εἰς τὸν οὐρανὸν πορευομένου αὐτοῦ, καὶ ἰδοὺ ἄνδρες δύο παρειστήκεισαν αὐτοῖς ἐν ἐσθήσεσι λευκαῖς
The apostles have been (presumably) gazing at Jesus as he rises into the sky. The final phrase introduces a new subject and verb:
καὶ ἰδοὺ ἄνδρες δύο παρειστήκεισαν αὐτοῖς ἐν ἐσθήσεσι λευκαῖς
The verb* is παρειστήκεισαν, part of the complicated conjugation of the -μι verb παρίστιμι. Παρείστήκεισαν is a 3-P, active indicative of that rare beast, the pluperfect.
(This is the only use of the pluperfect of this particular verb in the New Testament.)
Παρίστιμι has an extensive and complicated set of meanings, but in the pluperfect it generally has an intransitive sense: 'I stand, I stand near, I am present'.
The subject of παρειστήκεισαν is 'ἄνδρες δύο', 'two men'.
So two men stood by them, the apostles. 'Them' is indicative by the dative plural of the personal pronoun: αὐτοῖς.
The final words, ἐν ἐσθήσεσι λευκαῖς, are a prepositional phrase, describing something about the men - they are 'in clothing / apparel white'.
The feminine noun for clothing is the third declension ἐσθής, ἐσθῆτος, with ἐσθήσεσι being the dative plural. This is the only time this word is used in the plural in the NT.
The adjective λευκός, λευκή, λευκόν ('white') is the source for the English 'leukocyte' (white blood cell) and leukemia.
Here is a translation of the entire verse:
and as they were looking into the sky, he went, and - see - two men stood by them, in white clothing
* I should note that there are actually two verbs in this phrase. Ἰδού is properly a verb, although Thayer's Lexicon describes it as a 'demonstrative particle'. It can be parsed as the 2-S, aorist middle imperative of ὁράω, keeping in mind that the conjugative complexities of the several ancient Greek verbs of seeing (Βλέπω! Ὁράω! Εἶδον!) are beyond easy explanation. It has been used from Sophocles down to mean something like 'see! look!'