How to implement user registration and authorization from forum engine classes?

Question

User identification

There is a small html site that provides arbitration\service, orders are made by phone.
You need to make a member panel for information on the service, with registration and authorization + add a forum.
There was an idea not to reinvent the wheel, but to use classes from the forum engine for registration and authorization. The advantages are obvious: no need to "reinvent the wheel", the user immediately has the opportunity to write to the forum, a single nickname (login) here and there, a bunch of registration plugins through social networks.
Normal thought or not? What are the downsides?

Answer 1

If you do not want to reinvent the wheel, take a ready-made cms with an integrated forum and transfer the html site to it.