This is usually indicated in the event. Mostly, wear something in which you can sit, move, dance and lie down freely.
Through the breathwork and energy moving most people can get very hot. If you sweat easily, bring an extra shirt.
Take into consideration that you will share intimacy with yourself and possibly with others, being mindful of smoking and bringing a tooth brush can be comfortable for both you and your partner(s).
Sometimes it is explicitly indicated to bring an extra cushion, water bottle, notebook, sarong or oil.
Retreats. Training. Coaching.