How to make different opacity for different players?

Does anyone know if there is a way that I can make certain blocks invisible for some players and visible for others?

Use local scripts

There is something called SetLocalTransparency which is used for BaseParts, however everything done in a localscript is ran on the client, so if you set the transparency of a block in a localscript then that block will be different to everyone else, make sure you set the transparency when you need it to be set otherwise it will just be transparent for everyone and will just look the same for everyone