I agree that you mustn't take it to heart! Otherwise you will feel dejected and suspicious of them and won't enjoy the companionship they are able to offer.
I know how it is to feel 'left out' or 'not as popular' as others in a group. But I realise that a lot of this is a hang up from high school and I have to have a serious talking to myself when I begin to feel little and insignificant.
I then focus on having a nice chat and some company whilst doing childcare and really my NCT pals were a lifeline, especially when DS1 was very tiny and I felt completely out of my depth.
I think what you will find is that these meet ups are often pretty random anyway. So a couple will be chatting and suggest meeting up. Then one of them runs into another at a shop and mentions the meet up and on it goes.
And don't forget that you will probably all be suffering with baby brain to some degree or another so the group won't be the most organised in arranging meet ups with military precision and checking RSVPs etc.
I'm not belittling how you feel, really, but just do try and enjoy the group for what it is.
However if they seem to 'accidently' leave you out on a regular basis then they are probably not the sort of women you want to socialise with anyway. But don't worry too much if that is the case, there are plenty of other Mums you will meet through various baby activities and groups and you are bound to find someone you click with!