Sometimes in questions by new users (this for example) the comments go on too long and they should really be moved to chat. But new users don't have the 20 rep needed to write in chat. How can they be invited then?
Moderators can explicitly allow any user access to a certain room, but unfortunately there doesn't seem to be a way for non-mods to do this.
If no mods are readily available, then it's trickier. I guess the next best thing you can do is flag the question as "Other" with a message to let us know there is a lengthy comment discussion which can't be continued in chat without a mod.