If your kid has poor social skills, instead of telling him: "go play with other kids" or even worst "why don' t you play with the others", give him/her a specific mission, such as: "go and tell *that* kid to play with you, he seems like a good kid". It's a more positive approach and the child will not feel as self-concious about his difficulties.
A specific mission is a great idea and modeling good social skills is another great thing. Sit down with your child and a friend and play a game with them. If a proble arrises you can address it head on and in a positive way. Keep the game short and simple until your child has the skills and ability to play with another child for longer periods of time.