The question is a grammatical one, although the grammar cannot solve it as the construction is fundamentally ambiguous grammatically, in Greek as in English:
Salute Andronicus and Junia, my kinsmen, and my fellow-prisoners, who are of note among the apostles, who also were in Christ before me. (Romans 16:7, KJV)
16:7 ἀσπάσασθε Ἀνδρόνικον καὶ Ἰουνίαν τοὺς συγγενεῖς μου καὶ συναιχμαλώτους μου οἵτινές εἰσιν ἐπίσημοι ἐν τοῖς ἀποστόλοις οἳ καὶ πρὸ ἐμοῦ γέγονασιν ἐν Χριστῷ
As in English “famous among” does not tell you whether the person is part of the group.”Ronaldo is famous among Portuguese footballers” and “Ronaldo is famous among Juventus fans” and are two very different statements. Only context, not grammar, will tell you which is meant.
The word “of note” “notable” is only used one other time in the New Testament – of Barabbas who was a “notable”, or more correctly “infamous”, criminal. (Matthew 27:16)
Who were they?
Andronicus and Junia appear to be a couple, a male and female, and yet there is no record of them outside this verse – or in later early Christian documents – as having been apostles. There is some confusion (in Latin) that Junia was sometimes explained as a male name, but in Greek the name is female.
The term ‘apostle’ is a common Greek word meaning “one who is sent”, or messenger, but the way it is used across 81 times in the New Testament gives it a meaning in the church use that is not present in classical texts. The highest use of the term is for Jesus himself, the “apostle and high priest of our profession” (Hebrews 3:1). At the other end of the scale are two uses for messengers within the church: first unnamed “messengers of the churches” (2 Co 8:23) , then “Epaphroditus, my brother, and companion in labour, and fellow-soldier, but your messenger” (Philippians 2:25). The rest of the “apostle” uses are what we would expect – The Twelve, Paul, Barnabas, Timothy, Silas. The big name messengers, preachers, in the New Testament.
Episemos en in classical Greek.
The best known occurrence of the phrase “episemos en” (famous among) in classical Greek is line 104 from Euripides play Hippolytus, 428 BC. Language moved on from the Attic to Helenistic periods of Greek, but the sentence from Euripides is no different from Paul’s. Clearly the meaning here is that Aphrodite is revered by mortals, not a famous mortal.
Hippolytus :  Which? Careful lest your tongue commit some slip.
Servant pointing to the statue of Aphrodite : The goddess here, who stands beside your gate.
Hippolytus : I greet her from afar, for I am pure.
Servant : Yet she’s revered and famous among mortals.
Hippolytus : I do not like a god worshipped at night.
Ἱππόλυτος  τίν᾽; εὐλαβοῦ δὲ μή τί σου σφαλῇ στόμα.
Θεράπων τήνδ᾽, ἣ πύλαισι σαῖς ἐφέστηκεν πέλας.
Ἱππόλυτος πρόσωθεν αὐτὴν ἁγνὸς ὢν ἀσπάζομαι.
Θεράπων σεμνή γε μέντοι κἀπίσημος ἐν βροτοῖς.
Ἱππόλυτος οὐδείς μ᾽ ἀρέσκει νυκτὶ θαυμαστὸς θεῶν.
So “notable” like Aphrodite, or the greatest apostles?
So, back to where we started, the Greek here is ambiguous.
Either this couple, Paul’s kinsmen, were respected by The Twelve, Barnabas, Timothy, Silas, etc.
Or the couple occupied an eminent position equal to The Twelve, Barnabas, Timothy, Silas in the top ranking of major apostles, ahead of a much larger group of lesser “apostles” – that we have no record of.
The former option is the reading taken by the ESV. Some other versions take the second option.
Greet Andronicus and Junia, my kinsmen and my fellow prisoners. They are well known to the apostles, and they were in Christ before me. (Romans 16:7)