Even if you are not coconscious you can still talk to this new alter. Just talk to her as though she is listening. It doesn't matter if she doesn't reply.
Most alts have a need - perhaps this one has a need to share her stories. It is important to acknowledge the need, but also share your own need (ie, for her to slow down so you can, as a group, really *hear* what she needs to say.)
In my own experience alts get angry and antsy when they feel unheard, disrespected, ignored or invalidated. That is why it is so important to let her know that what she has to say or share *is* important to the rest of you, but the way she is telling it right now is too hard for the rest of the group. See if you can come up with some suggestions as to how she might share her story differently, in a way that the rest of you can tolerate.
It's all about working together. And remember that coconsciousness isn't always two-way... other alts can be much more aware of others within the group than you yourself are, and it is probable that this alt can hear you, even if you have no coconsciousness with her.
Start talking to her as if she can hear you. Get communicating!