In this context, a platform is an on-line service. The process of registering new users is known as onboarding. I do not know if describing such process as onboarding users to the platform or onboarding users into the platform.
In business jargon, onboarding is registering, assimilating, training, and setting up a new employee to be able to do their new job. It therefore adds little or nothing to the concept of simply registering a new user to the platform, unless there is also an element of training and mentoring new users as they register.
If it must be used for some reason (perhaps to do with corporate conventions that require this jargon), it seems better to register the users to the platform rather than registering them into and also on(board) it. One preposition seems sufficient rather than two that merely confuse and muddle.