I have a similar problem to @Sergiy (the answers already cleared it up mostly) that on import, users who have sent messages in the imported team/channel are not members. I also cant figure out how to edit the import of the team or channel to automatically add the users to that channel.
I had a few problems with this as when I added a new team called “all” and tried to enter it after adding myself to that team, mattermost was trying to redirect me to “all”'s “townsquare” constantly but this resulted in looping rejections as I was not a member of “all”'s “townsquare” channel yet. I ended up having to use the command line tools to add an additional public team called “Aa” (I had a theory I was being redirected to “all” townsquare as it was alphabetically the first team I was in) and add myself to it’s townsquare, as even when I added myself to “all” I was still rejected til I was able to get Mattermost to redirect me somewhere else. After that I was able to enter “all” fine. All the same, it would be nice to be able to add users to channels and teams automatically on import (especially adding them to the townsquare automatically!) to avoid this problem, so if anyone can direct me as to what to edit in the json files or add to them, itd be a great help.