I worked at at theme park doing street magic for a summer. At first it was a little nerve racking walking up to random people and showing them a few tricks. My show took on more of a busking format, but this is beside the point. I eventually got used to performing for random people. I now find it extremely easy to walk up to a group of people and show them stuff.
I usually introduce myself with "Would you like to see something interesting?" This rarely gets turned down, and I don't instantly get stereotyped as a "kids show performer," not that there's anything wrong with that, but it seems like most of the time people think that magic is a kids thing, and while this is true, it can also be something for older audiences. Plus, I'm a card guy and cards don't work well with kids, or vice-versa.
Also, I try to approach people who are meandering about, or who don't look like they have anything in particular to do. In other words, people who seem like they're just out to have fun. This increases your chances of being able to perform, and increases their chance of enjoying your show.
I recently did a quick little street show for some teenagers at a local beach. This was risky because there was a police officer standing right there and I don't know what our town's laws are about street performing. I didn't ask for money, and the officer didn't come talk to me, so all was good there. Anyway, I only had cards with me, so I started with the Biddle Trick, went into 2 Card Monte, and closed with Las Vagas Leapers. I left the small crowd completely stunned and later saw one of them telling someone else about the show.
I'm not sure where I was going with this, but I'll leave that last paragraph as an example of a good street show, I guess.
You asked if you should always go with friends. I would. I don't want to be alone when someone decides they really don't like me. Hopefully that doesn't happen, but it's always possible. Plus, I think most things are more fun with friends.
