Greek text is Nestle-Aland 27; English text is World English Bible:
16:19 δώσω σοι τὰς κλεῖδας τῆς βασιλείας τῶν οὐρανῶν,
16:19 I will give to you the keys of the Kingdom of Heaven,
"(to) you" [soi] here is singular
καὶ ὃ ἐὰν δήσῃς ἐπὶ τῆς γῆς ἔσται δεδεμένον ἐν τοῖς οὐρανοῖς,
and whatever you bind on earth will have been bound in heaven;
"you bind" [dēsē(i)s] is 2nd person singular
καὶ ὃ ἐὰν λύσῃς ἐπὶ τῆς γῆς ἔσται λελυμένον ἐν τοῖς οὐρανοῖς.
and whatever you release on earth will have been released in heaven.”
"you release" [lusē(i)s] is 2nd person singular
Here, Jesus is speaking directly and specifically to Peter.
18:18 Ἀμὴν λέγω ὑμῖν· ὅσα ἐὰν δήσητε ἐπὶ τῆς γῆς
18:18 Most certainly I tell you, whatever things you bind on earth
"you" [humin] and "you bind" [dēsēte] here are both 2nd person plural
ἔσται δεδεμένα ἐν οὐρανῷ,
will have been bound in heaven,
καὶ ὅσα ἐὰν λύσητε ἐπὶ τῆς γῆς
and whatever things you release on earth
"you release" [lusēte] is 2nd person plural
ἔσται λελυμένα ἐν οὐρανῷ.
will have been released in heaven.
Here, Jesus is speaking to "the disciples" (18:1) more broadly.
There remains the question about what this means for designation of "authority" in these two passages. In Matthew 18, the context is clearly that of discipline within the community, and the nature of forgiveness and exclusion. It is important to note that this is not directed even towards the Twelve, but (so far as we can determine from context) to Jesus' "disciples" more generally, which can extend well beyond the Twelve, and here would be designation for all who follow Jesus.
In Matthew 16, things are different and there is a lot at stake in these verses for the development of the the Christian church(es) in the centuries to come. Consequently, there is also a lot written on this passage that a brief Q&A on BH.SE will not satisfy! It is difficult to know how v. 18 relates to v. 19, for example: is the "rock" upon which the church1 is to be built
Peter himself? on the "revelation" which preceded? on the "binding/loosing" which follows? (Perhaps others will provide some comment/resources on this point.)
More broadly still, there is the nature of the "binding/loosing". Given the context of Matthew 18 in particular, my own sense is that this language should be taken in conjunction with the Lord's Prayer and its commentary in Matthew 6:
12 Forgive us our debts, as we also forgive our debtors. ...
... 14 “For if you forgive men their trespasses, your heavenly Father will also forgive you. 15 But if you don’t forgive men their trespasses, neither will your Father forgive your trespasses.
- It should be noted out that ἐκκλησία "church" occurs in the four canonical gospels only in Matt 16:18 and 18:17.