First, A multiple select list will only send all of the selected options in PHP if it properly named as an array (the name needs to end in ""). If it is not named to create an array, only the3 last selection made will be passed. the select tag should look like:
<select name="username" size="12" multiple="multiple">
using the implode function on an array will not create a separate record for each selection, it will convert the array to a comma separated list.
If you wish to save a unique record for each selection, you will need to hand code the insert, there is not a tool that will do this for you. You will need to code a loop through each index in the array, and execaute an SQL insert statement for each selected user name.
It will be easier to store the comma separated list in a single record.