I would suggest you also to subscribe to the kotlin mailing list. I don't use kotlin nor participate on the mailing list, but i think it is really important when exploring new technologies (and in particular not very mature technologies) to have a sense on how participative is the user community, their priorities, how diverse it is, how is the interactions with the mortal developers like us, because you will be there for some months/years.
My experience with the groovy community and resources like this mailing list is amazing. I have been learning something new on each new discussion thread and it is one of the few mailing lists that i keep following closely because i feel that gives me an added value that I won't find easily on other places.
Also I would suggest looking at the committers list from their source repository, it can give you and idea on who and how many are the kotlin developers, it they are all from Jetbrains or just one developer it would be a yellow alert in comparison with Groovy, because kotlin would have the resources concentrated, so if there is a change on Jetbrains business (priorities, financial, etc), the entire project is in risk. I like that Groovy is diverse in their core team and makes me feel more comfortable even on this transition times from Pivotal's stewardship.