Asking out a coworker you're already friends with

How would you approach asking out a coworker that already hangs with the same friend group as you? There was one girl that I talk to sometimes, so I can't say I'm deep in the friend zone, but she's already hung out with the same friend group as me a few times. So it's like how do I separate this hangout from all the other ones, as a date, if I have never been explicitly sexual towards her? Explicit? Implicit? We were making plans for just the 2 of us, I invited her over to my place, but she responded "aren't we already hanging out with {third friend}" (another girl who was in town and trying to make plans with the both of us), which tells me she thought we were hanging out as friends.