There are a couple ways in which one can manipulate the emotions and/or mood of a crowd and I have done both with slightly different results but either way the outcome was similar in that the mood of the room had changed to that which I desired.
One way that I will do this, and it is slower but more gentle and less...hmmm...invasive to those effected. I center my own energy within my core, picture it swirling like a tornado, brightening and becoming more potent, all the while concentrating on the emotion or combo of emotions that I wish to bring to those in the crowd. Usually it is something like excitement or enthusiasm laced with a bit of adventure and glee or (gasp...dare I say it?) sensuality. As I concentrate on these emotions and moods, I infuse that feeling into the swirl of energy that is growing brighter within my core. If you are the sort that sees emotions as colors then you'll know when the energy has been infused with the emotion because it will take on the color of the desired emotion. If you are more like me and just feel or taste the emotions then you will know that you have finished infusing your energy when you begin to not only be concentrating on the feeling you wish to give but are truly experiencing these feelings thoroughly.
At this point I know that the energy is ready to be released into the room. It seems there are as many ways to do this as there are indiviuals that have done it...in other words, choose a visualization that works best for you and feels right. I will share mine. My center, where I hold my core energy, is just below my heart and beneath my sternum. So to release this infused energy I visualize an opening of my heart chakra like a bright light and this energy flows out through it in a mist that expands as a cloud over the entire crowd. Then I allow the mist to slowly settle on everyone in the crowd and soak in. This is a gentle slow process but it allows the infused energy to be absorbed without dissipating into the environment and going to waste. If you were to just dump it all on them at once, the effect may be faster but a good deal of it would run off and never be taken in or benefitted from.
Now I mentioned that there was more then one way to do this and that the way i just mentioned was the gentle less invasive way. There is another more..influential way to do this but I must advice caution in using it as it does mess with the free will of others. Okay, I've put up my disclaimer now so here it is...
This is a more "hands on" and personal approach. Basically, it involves taking the time to focus on the crowd as a whole and really watch them. Who are the key players? Which people are the ones that the others seem to look to in determining the general mood? Who are the dominant personalities of the group? In short, who stands out? These people will be your tools in making your mood adjustments to the crowd because they project well and will be doing half of your work for you. In a sense they will be the amplifiers for the melody of emotion you wish to put out.
Once I have picked out my "amplifiers", I take the time to concentrate on them one at a time. Delve into them. Really tune into them and find what they are feeling. If it is necessary for you to make this kind of mental and emotional reading, then send out a tendril or two to make contact with this person. Sometimes it helps to spark up a conversation with them if one is new at this but with practice one should be able to do this entirely from the corner of the room without having to interact at all with anyone. Once I've gotten a vibe for what this person is feeling, I can see which direction they need a little push toward in order to get them closer to what I want the group as a whole to be feeling. Don't be surprised if this doesn't work the first time...free will is a tricky thing and you may find that at times it can be something like attempting to herd cats when it comes to getting a person where you want them to go the first time or two. But again, with practice it becomes easier.
Allow me to give an example of how this has worked for me in the past though. I was at a dance club and had noticed that the entire mood of the room was just plain flat, dull, borrrrring. I picked out my amplifiers and went to work on them. The first was a girl sitting at a table full of her friends who all seemed to be looking to her and noticing that she wasn't having a good time, they in turn were not having a good time. I focused on her, read her and peeked in on what was bothering her. She was lonely because her boyfriend had not been able to come along which in her view made everything more dull. So instead of just pushing the emotions I wanted the room as a whole to feel into her, I sent her a sense of comfort, whispering in her mind that he missed her too but wanted her to have a good time with her friends because she so rarely gets out. To her, she was merely remembering the words he had told her earlier that night but they just so happened to be just the words she needed to recall at that moment. At that point she slammed her hands down on the table and turned to her friends and said, "What are we doing just sitting here guys? This is a club, man! Let's get our dance on!" So you can see how focusing on each of your amplifiers one at a time will change the atmosphere in a room dramatically.
And that's pretty much all there is to it. Once you have helped give a little push in the right direction to each of your amplifiers, they'll do the rest for you and you can then just sit back and watch it all play out or get out into the middle of the crowd and soak up the good vibes. Cheers!