I'm not sure what the jabber admin page you're talking about is. Building the page with multiple text fields sounds pretty awkward though. Is there any reason why you couldn't accept a comma-delimited list for add/delete? Or when listing the buddies, have check boxes next to each item name and a "Delete" button on top and bottom? (Like message deletion in most webmail systems.) Other postings on these bboards have discussed UIs for gracefully administering various numbers of users. See
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0006eQ for one.