S.T.
i would work with this young introvert by understanding her personality and her triggers, and as much as possible honoring them and helping her understand herself. she's already doing a great job of expressing herself, and she's really very, very young. so start there, and gently guide her to keeping up her vocal expressions of her needs, but doing so more quietly and courteously.
don't fall into the 'embarrassment' trap. so many young mothers (i was one!) are very concerned about how other mothers view our parenting and are a little prone to lean toward doing what won't make us 'look bad.' but if you've got a kid who gets overwhelmed by too much social contact, you should recognize that and help her learn to set boundaries courteously, but without getting too hung up on how it appears to others.
and doing so within your own family is a great place to practice, so when she's on playdates she's got the tools.
she's too young to realize when her stress levels are mounting, but as her mom i'll bet YOU can see when she's starting to fume. start by talking to her about it in very simple terms when she's calm and cheerful.
'marigold, we're going to practice how to behave when you're playing with someone else and you get angry. sometimes you just want to play by yourself, and that's fine! but we need for you to be kinder and more courteous when you've had enough. let's come up with a signal that means 'time for marigold to play alone.' how about a peace sign? or wiggling our noses? but you may not yell at your sister or friends and stomp out. that's rude, and when you do it you're going to lose a privilege.'
and you can work on just what that consequence will be, but you don't want to overwhelm her with tons of new rules. just the relief of having an escape plan may largely eliminate the problem.
khairete
S.