No one in The Gang seems to have any friends outside The Gang. When Dennis inherits his mother's house and plans on using it as a "party mansion", everyone they think to invite is either dead or would have nothing to do with them because of something terrible The Gang has done to them. ("Dennis and Dee's Mom Is Dead") Dee has to trick The Waitress into going out for a "Sex In The City" night with her by saying she will give her a notebook Charlie keeps on her if she does. ("Who Pooped the Bed?") The Gang is forced to use Artemis as their sound person for a kid's pageant they put on even though she doesn't know what she's doing, because, as Mac says "We don't have a very deep bench". ("Frank Reynolds' Little Beauties")
The Gang has ended up causing much hardship to some of the people they come in contact with: Matthew Mara, aka Rickety Cricket, was an ordained priest before an alleged "miracle" at Paddy's Pub ("The Gang Exploits A Miracle") reintroduced him to the Gang, and soon he found himself in a downward spiral that soon reduced him to a homeless "street rat." Similarly, they have cost The Waitress her job ("The Gang Sells Out") and subjected her to various other emotional trials and tribulations. They even cause trouble for people they don't actually know: while trying to "expose" Bruce Mathis as a "terrorist", they end up destroying a total stranger's car by ramming it with Frank's "rape van" multiple times. ("The Gang Solves the Gas Crisis")
Members of the gang often create alliances against each other, sometimes also allying with others:
Mac and Dennis vs. Frank and Cricket: Mac and Dennis hunt, despite Frank's warnings. Frank decides to join forces with Cricket to hunt Mac and Dennis back. ("Mac and Dennis: Manhunters")
Charlie and Dee: Charlie and Dee team up to find the source of Frank's meat, eventually deciding that it must be human meat. They bring a homeless guy back to Charlie's apartment, where they prepare to eat him. ("Mac and Dennis: Manhunters")
Dee and Dennis vs. Charlie and Mac vs. Frank: Dee and Dennis go undercover to find a serial killer, first using Dee as bait and then later deciding to pretend to kill The Waitress. Meanwhile, Frank tries to prove that the killer is Mac; Charlie fulfills his desire to become a lawyer by defending Mac against the accusations of the Gang. ("Mac Is a Serial Killer")
The Gang all went to high school together [except for Frank].
Until 02x02, Mac and Charlie were not aware about Dennis' classification of their group as "The Gang".
Schmitty was the original fourth member of the gang and a roommate of Charlie's. Charlie despised Schmitty and eventually turned Mac and Dennis against him. Dennis kicked Schmitty out of a moving car and told him "You're out of the gang!" ("The Gang Gets a New Member")
The Gang has a ritual in which each member of The Gang gets a special day to themselves, during which everyone else must do exactly what they want them to do, without complaining (or else you get another day added on). The episode "Mac Day" takes us through a Mac Day, but we hear a bit about Charlie Day (which mostly involved hunting ghouls), and we see the beginning of Frank Day.