Me and my girlfriend have been talking about having a threesome with another girl for a while, but just wondering what you guys think would be the best way of finding a girl to come and get involved with?
Ah one of life's biggest dilemmas. How do a straight couple find a girl for a 3-way? Well, in most cases it's not easy. It's unlikely to happen spontaneously and will probably involve some embarrassment and rejection along the way.
My first general piece of advice is to put your girlfriend in charge of making the selection. If it's her first threesome and/or sexual encounter with another girl, it's likely she's quite anxious and concerned about how inviting another women in to your bed may affect your relationship dynamic. Having her in charge of choosing the 3rd alleviates a bit of the potentially threatening concept of you selecting someone who you think is hot and you wanna fuck.
Ideally you should begin with your girlfriends friends and your mutual friends. That way there is an already established relationship, trust, and knowledge of relationship boundaries. For some people knowing the 3rd is an absolute no go, and they would rather choose a stranger. But remember that it's always safer to fuck people you know and trust.
Your girlfriend probably has that one crazy friend who is up for experimenting, or maybe a bi friend who knows some girls who loves being the 3rd in couples threesomes. Your girlfriend will probably know where to begin with asking around, but might be too embarrassed to ask.
Another option is using internet dating, or a dating app like Tinder. You have to be really up front and clear from the outset that you are a couple seeking a lady for a 3-way, so you don't waste anyones time or hurt any feelings. Remember that gay and bi chicks on Tinder are probably really tired of people inviting them for threesomes, so be nice.
Whiplr is supposedly a kinkier version of Tinder, so you may find a bigger pool of potential 3rds. It also allows couples to join together, and lets you filter members by kink. So there's probably some sort of filter for females looking for MFF threesomes.
Finally, you could try the old fashioned way. Go out somewhere with your girlfriend, get a lil tipsy, and go scouting for babes together.