The social dynamics of bars and clubs are always interesting to observe because they often act as a microcosm of larger social interactions.
Very rarely will you see a person alone at a club, because most people are very self conscious, and intimidated to be by themselves. Frankly, I've found this method to be highly effective in meeting guys, and going home with them as well.
If you don't want to be seen as "unapproachable", the simple solution is to get over it and approach a guy you find attractive. Personally, this has always worked for me, and I've never had a negative experience -- I stand back and observe the guy for a short period to determine if we're a "match", and if I find we are, I talk to him to see if the interest is mutual.
If it is, we then lock into eachother, chat, and progress to the next stage, which usually involves kissing. I will say that it's much easier when you're starting off to try to approach a guy that appears to be alone -- it's less complex that way and you can almost immediately gauge his interest, without the interference of others.
If he's in a group, it can be a bit trickier, but you can still "get" the one guy you're interested in. It's all about maintaining a relaxed, confident disposition, while subtly clueing him in on your interest. At the same time, you must be cognizant of the group interactions to determine if he is with someone else, as well as the dynamics of the group -- sometimes, based on body language cues, you can determine whether or not a group is open or closed off to other people.
I've never understood why guys go to clubs with a group of friends, and then give off this closed off vibe. Then, they wonder why no one talked to them or why they did not get approached. The point of going out, I've always thought, was to meet new people.
1. Confidence -- head up, relaxed disposition, fluid movements, and slow down
2. Eye contact -- if a guy looks at you, DON'T look away. Flirt with your eyes, smile, and look down. If he's interested, he'll probably approach you.
3. Smile -- don't be afraid to smile.
4. Group dynamics -- if you're in a group, be open to outsiders, and if you see a guy alone who you find attractive, go up and and introduce yourself.
5. Approach -- again, don't be afraid to approach a guy you find attractive. Do you match up? Are you relatively of the same attractiveness? Chances are he'll be interested.
These tips I think will help you get over the stigma of being seen unapproachable.