Paul,
It would help if you can indicate what type of riding (roads, dirt, mix, etc.) you are interested in and what type/size of bikes, you and your mates riding skills and how many days you are going to be in Thailand/Chiangmai.
Maybe also good to know that in June the rainy season will have started and, depending on how much rain we get in June, this may affect the type of riding
Cheers,
Auke