Felicity worked at Queen Consolidated for Walter Steele. Walter and Oliver separately asked her for help leading her to believe Oliver was The Hood, and he brought her on board his team. Felicity now works alongside Oliver and Diggle as a trusted member of The Arrow team.
Felicity: What's going on? Ray?
Ray: Oliver Queen is the Arrow.
Ray: I have a 140 IQ and three PhDs, but it's pretty hard to insult my intelligence, but I'm pretty sure you just did. It all makes sense now. Your, ah, sudden disappearances, your apprehension of helping me with my mission. You've been working with the Arrow this whole time.
Felicity: You're not even flinching when he...
Sara: Pain and I came to a little understanding a few years back.