This has happened to me and I feel quite put out about it - I wouldn't have been able to go but they didn't know that so it is fairly obvious they didn't want me to go otherwise they would have invited me.
I found out they had gone the following day (Friday) when one of them invited me to be a 'friend' on Facebook and it said on her profile that they were going for tea the night before!!
Our department is seperated into 2 teams and I have always tried to bridge the gap between the 2 to avoid a 'them and us' situation but it appears that is what they want which is a shame.
(It may also be because they are all young and I am 10 years older - and they didn't want some 'granny' hanging round with them )!