My understanding here is that it's the former colleague who came over to sit with you whilst you were talking to an unrelated friend?
Without knowing the dynamics of the friendship groupings it's hard to really understand what's going on. For example, is this a group of women who all have a common interest in a maternity group, sports, an organisation or charity. How close are you all? Friends from school? Parents from school?
I think it's very likely that the person who introduced her colleague to the group felt it was a supportive community that would welcome her, and that perhaps this person, for whatever reason just hasn't gelled with everyone. They may struggle with social cues also... but I would also point out that if the colleague spotted (and recognised) you sitting with someone they didn’t know then you've already said that there's a fluctuating number of people who attend these meet ups.
Could it not be that the person recognised you and thought it was the start of the grouping (I understand the timing would be off here as it's early, but could they perhaps just have been eager to join?).
If I was meeting a friend before meeting a group of people then I'd have probably mentioned something on the group beforehand and also let my friend know my plans but you're also not wrong in not doing this either. I think it's just come down to someone eager to join and hasn't understood. Maybe just explaining the situation could help?
It sounds like you're annoyed at the woman having been introduced to the group, maybe she's sensing this and your reaction has confirmed this (even if your reaction was for a wholly different reason).
It's hard to know exactly without more context.