connect two wordpress urgent please

hello thanks for reading, I have a wordpress already working that we will call A, here the users register and have access to various topics, now I have created a site that we will call B, what I want to do is for users to enter from site b using the email and password of site A, since site b is only for validation since if it is not active in site A because it could not enter site B as I understand its plugint is ideal for this, I cannot find a manual on how I must put the data, I appreciate your enormous help congratulations on your work