Nicknames are funny. Who actually awards them? It depends on the group involved (in our family, most nicknames come from Mom, i.e. me). They aren't titles "awarded" by the head of an organization though - they can be agreed upon by consensus of the group, or each individual (as in the case of my family) can make up their own nicknames for other members. It's not just Mafia and families, either. Sports players can get nicknames, like "The Fridge" or "Sweetness," etc.
Another thing that marks in-group members is verbal habits, which can include everything from slang to special greetings to slagging (ritual put-downs, which are known by many different names depending on the social group). In a way, it's an honor to be able to put someone down safely.
I saw a great video of a teacher who had a different secret handshake for every single one of his students, and shook hands with each of them as they entered at the start of the day (video here). Freemasons were known for having secret handshakes also.
Clothing can also be a critical indicator of group membership. School uniforms, hats, or class rings, championship rings etc. Gang colors are an example of this. So are house colors like the ones in British Schools (and Hogwarts!). Fraternities have colors, letters, and a motto.
The creation/selection of these critical membership artifacts contributes to the sense of unity among the group. Groups have their own culture of shared interest.
Ideologies are also a form of in-group. Often people recognize members of the group from among people they don't know by listening to how they talk, as when people use dog-whistle words or phrases.
We chatted also about how we recognized members of the Science Fiction and Fantasy "tribe." Sometimes it's their clothes (a genre t-shirt, for example). Sometimes they are reading a book. Sometimes they have colorfully dyed hair. Sometimes the guys have long hair. Sometimes it's several of the above!
There were codes of dress and adornment favored by LGBT people, as when there was the habit of gay men wearing an earring in the right ear. In the case of groups like this, there can be potentially dangerous consequences for exposure, so it may be advantageous for the signals to be kept secret.
Groups can be formed on the basis of a shared job (as with unions, etc.) or a shared experience. Often, you will find that mothers will bond over describing their birth experiences. This becomes more complex in the case of transmen bearing babies.
Within groups, people often try to stratify themselves. Would that be indicated by scarier nicknames? Possibly.
Che talked about the witches in her book Tea Times Three, who were ostracized by non-witches. Each one of them is named after food! We wondered whether this was a witch thing, or a sisterly thing.
A really good source of conflict might be a person who is a member of two or more different incompatible groups being placed in a situation where they have to deal with one of each, and signal covertly to more than one person at a time, in different ways. When you are writing fiction, definitely think through what kind of groups exist and how they might be linguistically marked.
The identity of a group will reflect on the way the language use comes across. A character called The Nose might be very different depending on whether they are a perfumer, someone in an elementary school, a member of the Mafia, and alien, a mean girl, or something else. It's up to the author to create those associations. Set up the context, and take your time.
Given names for children are a way they can be involuntarily grouped. People change their names for various social reasons. Geisha take a performer name, as do people like Sting. Racism sometimes leads to people avoiding "ethnic" names. Lady Gaga chose her name to be deliberately ironic and to reference the song by Queen.
In Japanese, suffixes like -san, -chan, and -sama are used differently depending on whether you are members of a shared group (-chan and -kun are for insiders).
There are often nicknames that you don't want but which you are stuck with, and it can be problematic when nicknames are used out of context. A group may have a name for itself that is different from the public name it uses.
The use of a particular term can sound different when used by people in different social groups, as when the n-word sounds different used by white people (who are associated with racist contexts in which the word has previously been used) vs. black people (who have reclaimed the word in some ways).
License of intimacy often lets you use a nickname. Sometimes, external context like being at a diner can let you use a nickname.
Thank you to everyone who attended! This was a really interesting topic, and I enjoyed talking with everyone.