First thing is, please enclose your script in the script tags here so we can actually see the formatting. That's the button with the >_ icon on it.

Second, to solve the issue, you can use the printf command to print the array so it puts each name on its own line.

Here is your script with the script formatting in place and with the printf command included in the result output.

#!/bin/bash

#Script to detect if a computer has a local admin account on it with an UID of above 500
#Initialize array
list=()

#generate user list of users with UID greater than 500
for username in $(dscl . list /Users UniqueID | awk '$2 > 500 { print $1 }'); do

# Checks to see which usernames are reported as being admins.
# The check is running dsmemberutil's check membership and listing the accounts that are being reported as admin users.
# Actual check is for accounts that are NOT not an admin (i.e. not standard users.)
    if [[ $(dsmemberutil checkmembership -U "${username}" -G admin) != not ]]; then
        # Any reported accounts are added to the array list
        list+=("${username}")
    fi
done

# Prints the array's list contents
echo "<result>$(printf '%s
' "${list[@]}")</result>"

Last thing is, I'm not completely certain if your test condition will return the results you're looking for. Typically the != syntax looks for an exact match or in this case, non match. Since the dsmemberutil command isn't returning just "not" as its result, it may end up pulling in accounts that are in fact not admins. I haven't really tested your script to see about that though. If you find its not working as expected, you can change the line to look like this, which should work

if [[ ! $(dsmemberutil checkmembership -U "${username}" -G admin) =~ not ]]; then

First, thank you for the direction regarding the tags.
OK I left the old script in place for "User Admins" attribute and created the EA "User Admins v2" with your script. There are three accounts on this system, the test account is a standard account. The output was identical whether I used

#if [[ $(dsmemberutil checkmembership -U "${username}" -G admin) != not ]]; then

OR

#if [[ ! $(dsmemberutil checkmembership -U "${username}" -G admin) =~ not ]]; then

here is the output

I'd probably do it the other way around. Rather than go through every user and work out what their UID is and then check their membership, I'd get a list of the local admins and then remove those that don't have a UID greater than 500

#!/bin/bash

#Script to detect if a computer has a local admin account on it with an UID of above 500
#Initialize array
declare -a admin_list=()

# Test admin users and add users with id greater than 500 to array
function test_for_admins
{
        while [ $# -gt 0 ]
        do
                if [ `id -u $1` -gt 500 ]
                then
                        admin_list+=($1)
                fi
                shift
        done
}

# Get admin user list and pass to function
test_for_admins `dscl . read /Groups/admin GroupMembership | cut -d ":" -f 2`

# Prints the array's list contents
echo "<result>$(printf '%s
' "${admin_list[@]}")</result>"

exit 0

Thank you sean, We are getting closer, but I am still not able to get the Smart Computer Group logic to exclude known accounts. Excuse my script illiteracy, but would it be possible for the output to be filtered to exclude variables (known accounts) ? Resulting in just a users account (with admin) or a blank output (no other account with admin) ? Here is the current output

Thank you Sean, this works well appears that I only have to set my smart group to Admin Users "is not" blank.

Cool. You may want to mark it resolved for anyone else that may search in the future.

Just posting my version, slightly different use case... but might be handy for some.

@bentoms Careful with your script. You may get false positives. Eg.

#!/bin/bash

#What if the user is called David Min and is not a local account admin

adminGroupMembership="root admin"
loggedInUser=dmin

if [[ $adminGroupMembership =~ "$loggedInUser" ]]
then
        echo "User is admin"
else
        echo "User is not admin"
fi

exit 0

This is why I used an array and did exact matches.

This script it working well for me, however, I'd like to create a smart group based on this EA. At the moment, if it finds no known users, the result is blank. The problem with this is that I can't report on a blank EA field. Is there to modify this, so if no users were found it prints something like "No Admin Users"?

In response to @phillnz, I tried editing the original script by sean to print an output if the admin_list array is empty. I'm fairly novice at this but it seems to work. Feel free to suggest any improvements.

#!/bin/bash

#Script to detect if a computer has unknown local admin accounts
#Initialize array
declare -a admin_list=()
# Declare known admins
declare -a known_admins=('root' 'dsadmin')

# Add unknown admins users to array
function add_admins
{
        while [ $# -gt 0 ]
        do
                # Assume unknown until otherwise proven
                it_admin="FALSE"
                # Check admin aginst list of known admins
                # If known do not add to list
                for each_admin in "${known_admins[@]}"
                do
                        if [[ "$each_admin" == "$1" ]]
                        then
                                it_admin="TRUE"
                                break
                        fi
                done

                # If not known now add to list
                if [[ "$it_admin" == "FALSE" ]]
                then
                        admin_list+=($1)
                fi
                shift
        done
}

# Get admin user list and pass to function
add_admins `dscl . read /Groups/admin GroupMembership | cut -d ":" -f 2`

# Prints the array's list contents
if [[ $admin_list == "" ]]
then 
        echo "No unknown admin users" 

else 
        echo "<result>$(printf '%s
' "${admin_list[@]}")</result>"
fi

exit 0