Intergrating own website with wordpress

Hi Guys,

We have our own website where users need to login to post property etc.

We have built a library using a wordpress platform and now currently doing the same for our blog.

In the library and the blog we would like users to post comments, but we would also like that they use the same username as our main site.

Thus, they can move from one platform and another, using the same username/login details.

Can this be achieved?
Is yes how?
If not, what are our other options?

thankyou kindly,