How do I add a co-author to latest pushed git commit?

"Co-author" is not a git concept. It is a convention in commit messages used by some services, including GitHub. So, the solution is to edit the actual commit message with git commit --amend and add a line to the end:

Co-Authored-By: Name <name@email.com>

For commandline commit, I have created a prepare-commit-msg hook which needs to be placed inside .git/hooks/ and should be given executable permission. You can use up/down to navigate the list and space to select one or more author.

Usage: git commit -m "Message"
Note: Modify as you like as per your convenience

Ref: https://gist.github.com/smartameer/6c529bae770adbdd39bf895153564a34

#!/bin/bash

function prompt_for_multiselect {

    # little helpers for terminal print control and key input
    GREEN='\033[00;32m'
    YELLOW='\033[00;33m'
    RESTORE='\033[0m'
    ESC=$( printf "\033")
    cursor_blink_on()   { printf "$ESC[?25h"; }
    cursor_blink_off()  { printf "$ESC[?25l"; }
    cursor_to()         { printf "$ESC[$1;${2:-1}H"; }
    print_inactive()    { printf "$2   $1 $RESTORE"; }
    print_active()      { printf "$YELLOW$2  $ESC[7m $1 $RESTORE$ESC[27m"; }
    get_cursor_row()    { IFS=';' read -sdR -p $'\E[6n' ROW COL; echo ${ROW#*[}; }
    key_input()         {
        local key
        IFS= read -rsn1 key 2>/dev/null >&2
        if [[ $key = ""      ]]; then echo enter; fi;
            if [[ $key = $'\x20' ]]; then echo space; fi;
                if [[ $key = $'\x1b' ]]; then
                    read -rsn2 key
                    if [[ $key = [A ]]; then echo up;    fi;
                        if [[ $key = [B ]]; then echo down;  fi;
                fi
            }
        toggle_option()    {
            local arr_name=$1
            eval "local arr=(\"\${${arr_name}[@]}\")"
            local option=$2
            if [[ ${arr[option]} == true ]]; then
                arr[option]=
            else
                arr[option]=true
            fi
            eval $arr_name='("${arr[@]}")'
        }

    local retval=$1
    local options
    local defaults

    IFS=';' read -r -a options <<< "$2"
    if [[ -z $3 ]]; then
        defaults=()
    else
        IFS=';' read -r -a defaults <<< "$3"
    fi
    local selected=()

    for ((i=0; i<${#options[@]}; i++)); do
        selected+=("${defaults[i]:-false}")
        printf "\n"
    done

    # determine current screen position for overwriting the options
    local lastrow=`get_cursor_row`
    local startrow=$(($lastrow - ${#options[@]}))

    # ensure cursor and input echoing back on upon a ctrl+c during read -s
    trap "cursor_blink_on; stty echo; printf '\n'; exit" 2
    cursor_blink_off

    local active=0
    while true; do
        # print options by overwriting the last lines
        local idx=0
        for option in "${options[@]}"; do
            local prefix="$RESET ◻︎"
            if [[ ${selected[idx]} == true ]]; then
                prefix="$GREEN ◼︎"
            fi

            cursor_to $(($startrow + $idx))
            if [ $idx -eq $active ]; then
                print_active "$option" "$prefix"
            else
                print_inactive "$option" "$prefix"
            fi
            ((idx++))
        done

        # user key control
        case `key_input` in
            space)  toggle_option selected $active;;
            enter)  break;;
            up)     ((active--));
                if [ $active -lt 0 ]; then active=$((${#options[@]} - 1)); fi;;
            down)   ((active++));
                if [ $active -ge ${#options[@]} ]; then active=0; fi;;
        esac
    done

    # cursor position back to normal
    cursor_to $lastrow
    printf "\n"
    cursor_blink_on

    eval $retval='("${selected[@]}")'
}

exec < /dev/tty

echo 'Please select from the authors list:'
AUTHORS=$(git shortlog -sce | cut -c8-)
AUTHORS_LIST=()

while read -r line; do
    if ! [[ "$line" =~ ^.*\|.*$ ]]; then
        AUTHORS_LIST+=("${line}")
        OPTIONS_STRING+="${line};"
    fi
done <<< "$AUTHORS"

prompt_for_multiselect SELECTED "$OPTIONS_STRING"
MESSAGE="\n\n"

for i in "${!SELECTED[@]}"; do
    if [ "${SELECTED[$i]}" == "true" ]; then
        MESSAGE="${MESSAGE}Co-authored-by: ${AUTHORS_LIST[$i]}\n"
    fi
done

exec <&-

sed -i.bak -e "1s/$/$MESSAGE/" $1