How to remove user from a group in linux

For example:

$ id -nG i88ca

i88ca wheel postdrop

To remove i88ca from postdrop

$ sudo usermod -G i88ca,wheel i88ca

Put after '-G' a list of supplementary groups which the user is also a member of. Each group is separated from the next by a comma, with no intervening whitespace.  This is because if the user is currently a member of a group which is not listed, the user will be removed from the group. This behaviour can be changed via the -a option, which appends the user to the current supplementary group list.

See also:

How to add existing user to a group in linux