Here's the situation. I met a few people through meetup.com, and we all went on a hike. One of the people from that group (call her A) then had a little cocktail party where she invited this cute woman from that same group (let's call her B). B and I didn't have any interaction at the hike but we chatted when A had us over for cocktails.

While we were at the cocktail thing, B took a picture of us together and she sent me the picture (so I got her number without asking for it). A, B, and I made plans to do something at my place sometime soon but we haven't done that yet.

Now, I want to ask B out but I'm not sure what the best way to do this would be. I'm relatively new to this so kind of lost. Please advise.