If you need to wear diapers for bed wetting at a sleepover you can put the diapers or diaper into a backpack or anything else that you're bringing clothes to the sleepover in and then change into the diaper in the bathroom before bed.
Then just place the used diaper into a plastic bag and put it back into your backpack to dispose of at home after the sleepover.
You can hide the clean diaper in the backpack inside a pair of jeans or shirt so that it won't be easily seen if you open your backpack for something else during the sleepover in front of the friends.