set fd [open /etc/group r]
while {[gets $fd line] >=0} {
    lappend groups [split $line :]
}
close $fd

set index 5

# tcl 8.3
set name [lindex [lindex $groups $index] 0]
set gid [lindex [lindex $groups $index] 2]
set users [lindex [lindex $groups $index] end]
puts "$name: gid $gid, users $users"

set element [list $name x $gid ""]
set group [lreplace $groups $index $index $element]

#tcl 8.4
set name [lindex $groups $index 0]
set gid [lindex $groups $index  2]
set users [lindex $groups $index end]

lset  groups $index end $users