If i'm interpreting this correctly, you want a list of 20 posts down the page, with a select list next to each post, containing all of the members.
submitting the form will append 20 rows to the postholders table ?
If that is the case, then the insert should occur in a fraction of a second, and is not the cause of your timeout.
to test, i reproduced your database tables and a multiple insert page.
you will need to rename the fields and connection parameters, but it should give you what you need.
page is a bit ugly, with no formatting, but should get you going in the right direction