I have 1 new invite in my iCloud Photo Sharing (Shared tab in Photos app).
Will this person be able to see all my photos if I accept this?
iCloud Shared Photos Streams are different from your personal My Photo Stream.
Creating a Shared Photo Stream consists of choosing specific photos that you would like to share, naming the stream, and choosing who you would like to share with.
If you are recieving this invite, it means that someone else has chosen specific photos and/or videos that he or she would like to share with you. Accepting this invitation will simply grant you access to the photos that they wanted to share with you. After you except, you may view them, 'like' them, comment on them, and contribute any of your own photos or videos to the stream (but only if you want). No-one on the stream will have any access to any of your own photos that you have specifically added to that Shared Photo Stream.
In short, no, they will not have access to your own photos.