Here is a very straightforward string processing script that will print out the list of numbers you are looking for based on your description. Where I am printing out the numbers, you will want to capture those into an array or other structure depending on what you want to do with the data.
<cfscript>
memberOf = "CN=AG-880-ExpenseReports,CN=Users,DC=alcco,DC=com,CN=HTTP Users,CN=Users,DC=alcco,DC=com,CN=WA Houses,CN=Users,DC=alcco,DC=com,CN=MTViewMeadows,CN=Users,DC=alcco,DC=com,CN=ALC0169,CN=Users,DC=alcco,DC=com,CN=ALC0069,CN=Users,DC=alcco,DC=com";
memberOf = Replace(memberOf, "CN=Users,DC=alcco,DC=com", "", "all");
memberOf = Replace(memberOf, ",,", ",", "all");
memberOf = Replace(memberOf, "CN=", "", "all");
memberArray = ListToArray(memberOf);
</cfscript>
<cfoutput>
#memberOf#<br/><br/>
<cfloop array="#memberArray#" index="i">
<cfif Left(i, 3) eq "ALC">
#Right(i, Len(i)-3)#<br/>
</cfif>
</cfloop>
</cfoutput>