How to find out people interested in the same paper and discuss?
Professional societies often have mailing lists and a request there is probably an appropriate item. 

Departments sometimes have bulletin boards on which you could post a request. 

I found it very useful (before retirement) that our doctoral program had an internal mailing list to which every student and every advisor was subscribed. In fact, there were several lists, some specialized but one for everyone. Communication on such a list is very useful and I encourage others to establish these if they don't already do so. Our program was small, so a single overall list made sense. In a larger place it might be necessary to do this per specialty. 

But, don't just look for such things as a place to get help. Consider it as an opportunity for sharing. If you give you can get, but if it is just a request for help, it might be better directed at the faculty - say an advisor. 

But if you are faculty, then work to establish cross-university collaboration efforts so that like minded people have a way to communicate ideas. Having a wide range of collaborators is a good thing for most academics.