This is usually a totally unfair trick that women will play as long as they either dislike you or aren’t yet sure they do. They’ll invite a pal along to help size you up or to take the pressure from her needing to speak to you.
Say a date is established by one to meet a pleasant dude for a glass or two , so when you arrive, there’s another guy communicating with her. Or among her girlfriends shall there be keeping her company. That’s always a most unwelcome intrusion and a good signal that you can escape there. At worst, it might degenerate right into a two-way conversation, with you because the odd man out.
Once you see that she’s not by yourself, say that something came up and you also need to cancel. There is nothing to regret about causeing this to be move – you are not on a romantic date if there is a third wheel!