Yes. Once a tag has been registered to an account, you can easily share it with any other XY user. You can decide which people can see the tag when it is in range, follow the last known locations, and even get alerts when it moves out of their range.
XY will ship with a battery included, and when it is time to replace it, it takes a CR2032 coin battery. We estimate that the battery of XY will last approximately 6 months of normal use. This is not the absolute length of the battery (shelf life of up to 5 years), but the life when powering XY.
The Bluetooth signal on the tag broadcasts to approximately 150 feet. The range will, however be shorter if you are in places with heavy physical interference, such as a medieval castle, or other places with thick heavy walls.
XY is water resistant in that you can absolutely use it in normal day to day life without worrying about getting it damp. However, we do not suggest you try to place XY on the wreckage of the Titanic, as the design is not 100% water-proof, and you should probably not be that far underwater anyway. You might get the bends.
Don't see the answer to your question? Ask the project creator directly.