Giving a tip is completely up to you and there are no rules on how much to give. It is a custom to give a tip and at least a small one to thank the guides, driver and trekking team members for their services. Below is an idea of what you could give, it will always be appreciated if you give more.
If you are happy with the service of your guide, each person from the group can tip him 20 yuan per day. For example, if you are a group of 4 people that travelled 10 days, you may give 200 yuan each, so he will receive a 800 yuan tip from the entire group. For the driver you could give half of the amount you give to the guide. For the trekking team, the group together can give 15 yuan per day for each trekking team member (cook, helper, yakmen or horsemen).
For the guide and driver it is OK to give the tip in USD or EUR, but for the trekking team members it is better to give only Chinese yuan.