You might double post this on the special needs boards.
My DD is on the autism spectrum and had special services at school but our process was different because she already had a diagnosis before starting school. After she started college (yeah!) I got a job working with sp ed kids in a a school, and I'm now working on my sp ed teaching certificate.
The letter is written to make sure that you know your rights and know that you CAN bring other people if you chose. There are federal laws and a host of case law that govern the entire process. There have been issues in the past and parents rights are now clearly defined and the school is required to let you know what they are.
As a parent, I found it easiest to do the meetings with just me. In some families, mom and dad do the meetings together. The school I work at has a high Latino population, and some of them bring extended family. But for me, it was easiest for it to be just me (partly because my DH was in so much denial that it made it difficult to get anything done at the meetings).
You haven't said much about what has happened before this to get to this point, so it is hard to know what direction this is going. Part of the process is to do "pre-referral interventions." That means that they try making different accommodations in the classroom without going through the special education process to see if the issues the child is having can be dealt with in an easier way. I suspect that this meeting is to discuss what they've tried and how it has worked out.
A difficult thing about this process is that sometimes schools and parents doesn't see things the same way or agree on what is best for a child. Sometimes, parents see their child struggling and want them to get more help, but the school doesn't feel they qualify (BTW, if you feel that your child needs an eval, they HAVE TO evaluate). Sometimes, schools see children struggling and feel that they would be helped with a diagnosis and intervention, but the parents don't want the child labeled or to receive services. If either of these is the case for you, then I'm sorry (because both sides of that are rough), and I suggest you listen to what they have to say, ask questions, and think it over. One of your rights is to be part of the decision making process. You don't have to agree to anything at the meeting. If you do agree to an evaluation at the meeting, it just an evaluation. You aren't agreeing to any services. Just the evaluation.
Take something to write with in case you want to take notes.
If your son has other therapists, specialists, evals, etc., then you might organize that information into a binder and take it with you. If this is a whole new thing, you'll want to keep copies of everything.
I always found the meetings very emotional, and I cried. There was nothing shocking for me in them -- my DD's development was off from the time she was an infant. None the less, I found sitting around a conference table and talking about it rough. We were always lucky and had GREAT teachers, specialist, principal, etc. If I could go back and change one thing, I would give myself a greater sense of peace about the ways in which my DD is different from most people.
Here is a web site that does a nice job explaining the process: