Left out

At work there's a group of women (most above 40) that always leave a few of us women out when they arrange nights out with old colleagues we know too and haven't seen in a while. We all seem to get on during the day but they become really secretive and whisper about a night they've planned without us. I don't get why people are like that, it's really childish. Anyone else experience this sort of sh*t at work?