You've got two choices: If the comment train gets long enough you'll be prompted to automatically create a room for both of you. You'll both get invitations and life's pretty easy from there.
There is a manual way; unfortunately it's still kinds of difficult:
- Send user a link to the room where you want to chat. Note, you must be an owner of the room (create a new room or Bug-A-Mod for The Comms Room).
- After the low-rep user has "entered" they'll get the notice about "Must have 20 Rep...". You will see their icon in top-right; hover over the icon and note their User ID (usually a 5 digit number).
- On the "Room" menu (also top-right, just over the user icons) select "Control Access"
- On the next page, there's a section for "Explicit write access", click the "Add a user" button, enter their User ID from above, click "grant".
After you've done it, it's not that bad, but almost nobody would guess that.
PS> Don't worry about creating superfluous rooms, they auto delete after a couple weeks of inactivity.