I would add living people to my tree, but I am the only person who sees it. It is not published anywhere online or given to anyone. Any time I do pass on information I do not include any recent generations.
If you have your tree online, you can just write 'living' rather than mentioning names.
I would not like to be included on anyone's public tree, but I'm sure some people would not mind.
And of course you can try contacting them to ask for permission, and that also applies to the relatives you know - just because you know them don't assume that you can include them on an online tree without their permission.
