Informal:
* Guys
* Fellas
* Bro(s)
* Man(men)
* Mates
* Buddies
* Chaps (British)
* Blokes (British)
* Gents
More formal:
* Gentlemen
* Men
* Individuals
* Persons
* People
Slang:
* Bros
* Homies
* Dawg(s)
* Boys
The best synonym depends on your specific situation.
For example, "guys" is a good general-purpose synonym, while "bro(s)" is more informal and friendly. "Gentlemen" is more formal and respectful.