How to approach a researcher regarding his latest publication?
I have some considerations about their results, parallels with our work, ideas for future research etc.


So use this as a starter. Write/ask about their paper and get a discussion going. In my experience this will very often automatically lead to a collaboration of some kind, like a follow up paper or a whole joint project.

As an alternative I would recommend that you talk to your supervisor, what he thinks about the group and if he would think starting a collaboration would be worth a try. Maybe he will then start to initialise the communication.