It's another casual way of saying "guy". You can use "fella" in the same way that you would use "guy", or "fellas" in the same way you would use "guys". It's the shortened form of the more proper word "fellow". It's kind of an old-fashioned word, but I still hear it used sometimes. If I were talking to a group of females, I wouldn't say, "Look, fellas" or "Hey, fellas!" Though, I may do that if I were casually addressing 2 or more males. If I were talking to or referring to a mixed group of males and females, I still would probably just say "guys", "y'all" or "folks" rather than "fellas". I also would not refer to a female as a "fella". In the singular, "fella" refers specifically to a male. Also, use of the word has no romantic connotation.
- "Look fellas, I know I said I'd go out with you all tonight, but I'd really rather stay home."
- "My daughter brought this young fella over to the house the other day to watch a movie."
- I could easily imagine an old man in a rant, fussing and saying something like, "You've got all these young fellas out here causing all sorts of ruckus and chaos in these streets! Not in my day!"
- I could also imagine a swindler leaning in to talk to some guys who are skeptical of what he's offering, saying something like, "Fellas, fellas, fellas... Listen, this deal could be great for everyone!"