The new girl is never shown in the novel, manga or anime. The new character is not important either, it just goes to show that Saori found another person to join their group and things are going well in the otaku circle. Also, they never said that it was a girl, they just said a new member is joining.
To your opinion of Ayase joining, it could be but it would be very unlikely because
- Even though Ayase did say that she will allow Kirino to keep playing eroge, but to my memory, she did say that one day she will save Kirino from eroges and that she'll never play something as lewd as that.
- If Ayase was joing the otaku circle, you would think that Kirino would know about it since they are really good friends.
- Kuroneko's relationship isn't very good with Ayase.
Those are just my opinions though and I can't comment them.