Is this your first time as a stage manager attending a casting? Depending on the director, theater, and show, stage managers are utilized in various ways. Helping organize auditionee information, and aiding in whatever audition needs (such as passing out scripts, keeping notes on who is who, etc).... all that. Depends on the director whether you get input on who is cast. I've always had input.
In situations like this, it's fair to ask what is expected. I've been in a similar situation and I've stated, "In my previous experience, I've done this and this during casting and auditions. Because each theater has a different culture and process, what does *this* theater expect? I just want to make sure we are on the same page." I've approached it firmly and professionally without coming off as "uh...I don't know what I'm doing." It's about that communication since each theater functions slightly differently from time to time.
Hope this helps!