You can’t share your welcome guide until the sitter has accepted your formal offer of a sit. It your listing still shows the dates of the sit, you don’t have a confirmed sitter and can’t share your welcome guide.
If you do have a confirmed sitter, you can share your welcome guide once. After that, the sitter is notified if you make any changes to the welcome guide. At least that was what I was told by sitters when I asked a similar question to yours.