It can definitely be tough for an Aspie to make new friends, especially when they don't always respond appropriately to social cues, or know how to react in social situations. The best thing to do is find out if there are any support groups in your area. The Autism Society, or Easter Seals (or other special needs groups) often host "teen socials" and/or other activities for kids that can really be a great way to meet others, and practice social skills in an environment that is understanding of their unique needs. Once they gain confidence there, then they can apply those skills to other areas.
Not Too Many 1. One on one is best, too many people (2-3) can make them feel overwhelmed, so the introduction of another person must ne very gradual.
Prepare with Instructions 2. Show them friend making scenarios with pictures and text, so they know how to respond to that specific social friend making setting.
You can also read to them or just talk to them about how to make friends a bit each day, focusing only on one step of making friends at a time. Do this slowly and patiently until they appear to feel comfortable with the idea.
Bonding through something Familiar 3. Place an object or food they are familiar with or love, (like chocolate or a guitar, etc) and have the new friend have it near them and offer it to the Asperger's person or have the new person use the favorite object (guitar or other object) with the Asperger person in the room.
Go Slowly 4. Do not push, force, or otherwise cajole the Asperger person into any relationships or they will most likely withdraw even further into themselves. Go slowly and methodically and with great gentleness and patience.
Have Fun 5. Do something you know the Asperger person loves to do, listen to music, go on walks and slowly add another person to that setting.
(Example: have the new friend Joe walk by you and your Asperger's friend while you are on your walk and have Joe stop to chat to you for a few minutes (not to your Asperger's person unless he/she addresses Joe), then do this again the next day and try to include your Asperger's person in the conversation, but do not push.
Different Brain Pattern Responses The Asperger's brain is not the same and too many people, too much stimulation can sort of shut their brain off so to speak and they will retreat out of a need to survive, so proceed very slowly.
Go slowly and patiently and you may be pleasantly amazed and surprised.