If it already has grout you need to scrape off the gunk and seal it a few times with tile sealer to assist in making it waterproof. You would need to do that over and over again to have hope and prayer that it could be waterproof.
Grout is not normally waterproof, it absorbs water like concrete, but facilitates major water runoff to a drain. The tiles should be sitting on an actual waterproof membrane which is true waterproofing.
If you’ve got a leak and it’s been happening for a while, I’d recommend pulling the tile up entirely and doing a repair and proper waterproofing solution. Not tile.
You need a specific outdoor weatherproof tile grout and a specific adhesive.
But if you have a leak that’s coming all the way in you’ll need to look at the structure underneath the tiles, ideally, there should be a weatherproof layer between the tiles and the surface they sit on because relying on the tiles (which might be porous) and grout can lead to problems.
KASTAR Focus on ceramic tile grout for 30 years, There are a variety of sealants for you to choose from.
If you want more information about KASTAR, you can contact KASTAR When you are free. KASTAR will be at your service anytime!
