The most important person at a fitting is the bride. I have had some come on their own, with just one person, or an entire team of people. The best advice i can give, keep it simple. A fitting area can usually only fit a few people comfortably and have there still be room for a train to be spread out and for a seamstress to maneuver around.
If you know someone has a negative disposition, leave them at home. A fitting should be a happy experience, as a bride, you don't need someone making it a gloomy time. For anyone who attends a fitting as a guest of the bride, please don't touch the dress. While a seamstress is working we need things to remain in the same position until we are ready for the bride to adjust.
If it is a rainy or snowy day I may ask everyone who is attending the fitting to take their shoes off. The reason for this is to not track the dirty water around for the bottom of the dress to touch.