So recently i've had 2 people whom I have never talked to before nor seen them in the dissucusions i've been in (meaning I've basically come in contact with them in anyway) and I've been wondering what access they have when they add me as friend?? also I already know how to stop getting messages about people adding me as their friend
They would have no access, neither will you. The things you can do or they can do is make private shiritori and add you there or vice versa and sending the renshuu gift cards ( the new recent feature).
They friend you by clicking on your name (wherever they may see it), which sends them to your profile. In your profile there's a little "add friend" icon (you can do this by clicking my name if you want to see how it works). However, adding you as their friend doesn't give them any privileges other than what Seeji mentioned above, as well as seeing you in friend-based leaderboards.