Grout can be a diy project. Unless it was a product like Mapei Flexcolor CQ (which is a premixed grout), you will have to remove about 2/3 rds of the grout in the areas that need to be fixed in order to add new grout. Maipei Flexcolor CQ sticks to itself - even after it's set - so, you are able to simply add more grout where needed without removing any of the old grout.

There are YouTube videos on how to do this. I had two TINY spots I was going to repair - but then decided to renovate the entire bathroom.

FYI - some "holes" along the tub line (or premade shower pan) are intentional weep holes. Some tile installers don't put these in - but old school installers do to allow any water that gets behind your tiles to have a place to escape. Just be careful to not to fill any weep holes that were left intentionally. If you can't determine if you have weep holes, you can post photos on here and ask the tile pros.