Today there was a debate whether "confraternizing" was a word. It is important to the story that you know that one of the disputants is from Brazil. My answer:
Spellcheck is usually irrelevant. It's just not that comprehensive. When a word appears in a dictionary, it does not imply that all forms of the word are acceptable. You can check later in the entry to see if confraternize, confraternization, or confraternizing is also listed. In this case it is not, suggesting that these are probably not English words, though "confraternity" definitely is.
However - and this is where it gets interesting - it is usually worthwhile to google a word and see if someone has ever used it, and if it might indeed be acceptable in some circumstances. Googling the various forms of the word reveals that it is used in Portuguese, especially in Brasil. There are uses of it in English, but these generally fall into two categories: translations from Portuguese, where the translator clearly thought the word would be understood in English; and jocular uses, usually in quotation marks, such as "going to a pub and confraternizing with the locals." In these cases the writer's tone seems to indicate that he is making up a big word for humorous purposes.
There is an interesting exception to all this. The words "confraternizing" and "confraternization" are used in English translations from the Latin in a few places, describing the behavior of opposing armies having friendly contact with each other in wartime. This would suggest that the word existed in some form in Latin, which is how it got into Portuguese. In the translations from both Portuguese and Latin, the word seems to describe not just mingling, but distinct groups mingling with each other. This is not apparent in the English jocular senses.
Usage: I would use it only in a jocular sense, and I would preserve the distinction of mingling groups rather than individuals. I would not use the word in a formal setting.
Aren't you glad you asked?