How to find out people interested in the same paper and discuss? I am reading an academic paper, but I cannot understand a lot of things in the paper, especially some proofs. If I ask someone else who is not reading the paper, or who is not studying the similar research field, then he/she might need to spend much time and may still turn out to not understand the paper either. So is it possible to find out people who are also interested in the paper or have read the paper, then to create a discussion group? I think this will definitely accelerate my understanding of a paper and make research easier. 

Most of the time, I find out that I am the only one in my research group to study on some problems, and it is difficult to discuss with others on specific research problems. How to seek help from others who have different research topics? I do not want to waste other people's time if they cannot help eventually.
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.