Traditionally, there are two ways to set up the default group assigned to a new user. The first is to have everyone assigned to a single users group. This will allow all users on the system access files and directories with group read (and execute) permissions, and write to files with group write permissions.
The other way is to give each user his own groups, usually with the same name as his user ID. This is the default in Debian, and this scheme is often referred to as user private groups. Ifuser A wants others to be able to access certain files or directories via the group permissions, someone with system privileges that allow group modification must add the appropriate user IDs to user A's group.
At one point, a single users group was common practice. Best practice at present, though, is to use user private groups.
This is similar to the most common way to handle developer's access to each other's files. In this case, though, a new group, such as web-dev or whatever you feel is an appropriate group name, is created and the developers are assigned to it as necessary. A similar technique is used for some administration tasks as well. For example, an ftpadmin group that is allowed to modify the FTP server directories and files in ways that the normal (and especially the anonymous) ftp user cannot. Careful creation and assignment of groups can facilitate proper user access to files while preventing undesired access or modification.
User and group maintenance is handled by the standard command-line functions, such as useradd, usermod, userdel, groupadd, groupmod, and groupdel, all documented in the passwd package. Of course, there are GUI utilities available for user and group maintenance as well.
