How does grinding start...does a guy just put his hands on your hips and start grinding on you?

How the grinding starts depends largely on the dance environment. In some venues, it is considered necessary to verbally ask permission. I can offer my experience from large urban dance clubs (Philadelphia, in my case). In these venues, there is typically a large, crowded dance area at the center of the venue in which guys will begin grinding girls without asking. Typically, they will place their hand on the girl's hips and linger there for a second before making pelvis-ass contact. This would be your opportunity to repulse him if you are not interested. Generally, you can signal your interest in grinding by dancing in a grinding motion by yourself. If you are with a group, you are more likely to be ground upon if your rear end is facing out of your "dance circle".