Chromium Portable has a discussion forum at Google Groups. If you have any question, you can post it there. If you want to report a bug, see also the next section.
If you are posting the first time, there may be a small delay (usually less than one hour) before your post appears, because the moderators (i.e. I) have to approve the unrecognized address. After the first or second time, there should be no more delay.
Since your message will be sent for a large group of people, it is useful to observe a special etiquette when sending it. Basic help about this is at the Mailing List Etiquette FAQ. See also How To Ask Questions The Smart Way.
Before Reporting Bugs
Before doing anything else, check if the problem is not with Chromium itself, by trying to reproduce it in a non-portable Chromium or Google Chrome. If you manage to do it, please report it to the Chromium project instead. If you don't have Chromium installed locally, you can try to search in their bug tracker.
Once you're sure the problem is in Chromium Portable, search the forum for existing discussions about your problem. If there is one already, feel free to add more info to it.
If you really discovered a new bug in Chromium Portable, you can create a new forum topic. Remember to write a meaningful summary: not just “I found a bug”, but e.g. “Error when running from UNC paths” instead. Be as detailed as possible in the description; the more information we have, the easier it is to fix problems. Which Chromium Portable and Windows version are you using? What did you do when the problem occurred? How can we reproduce it? Did you changed any options from the default?