If you're going out into the hills it's important to make provision for your own safety and that of the people you're riding with. Make sure you know some basic first aid and at least one person is carrying a suitable first aid kit - Such things are available from every good outdoors shop. High hill and mountain weather can change rapidly and suitable caution and preparation should always be taken, if the worst happens you don't want to be underprepared and in a survival situation.
Take good warm waterproof and windproof clothing. Good quality jackets might cost a lot but they keep the elements out and keep your body core warm and dry. Good gloves and a thin hat to wear under you helmet are also invaluable pieces of kit that don't take up much room. Make sure someone knows where you are going and what time you expect to return.