Org-mode's capture templates include a %-escape for executing arbitrary elisp, which we can use to do practically anything. Here's how to use it to get what you want:
First, here are two functions that build a dynamic completion table. The first one just finds all matches for a specified regexp. The second uses the first to specifically get the sort of items you're looking for.
(defun my/dynamic-completion-table-regexp (regexp &optional group buffer) "Return alist containing all matches for REGEXP in BUFFER. The returned alist contains the matches as keys, and each key is associated with a nil value. If the optional parameter GROUP (an integer) is supplied, only that part matching the corresponding parenthetical subexpression is taken as the key. If BUFFER is omitted, it defaults to the current buffer. Note that the entire buffer is scanned, regardless of narrowing." (let ((buffer (or buffer (current-buffer))) (group (or group 0)) table match) (with-current-buffer buffer (save-excursion (save-restriction (widen) (goto-char (point-min)) (while (re-search-forward regexp nil :noerror) (setq match (substring-no-properties (match-string group))) (add-to-list 'table (list match)))))) table)) (defun my/dynamic-completion-table-by-field (field &optional delims buffer) "Return alist containing all entries for FIELD in BUFFER. Look in BUFFER for all lines of the form \"FIELD: TEXT\", possibly with leading or trailing whitespace. If DELIMS is omitted, consider TEXT as a single item; otherwise, it should be a regexp matching the delimiters between the items inside TEXT. Empty items are discarded, and whitespace surrounding the delimiters is removed. The returned alist contains one key for each item found; all keys are associated with the value nil. If BUFFER is omitted, it defaults to the current buffer. Note that the entire buffer is scanned, regardless of narrowing." (require 'cl-lib) ; For `cl-mapcan' (let ((table (my/dynamic-completion-table-regexp (concat "^\\s-*" (regexp-quote field) ": \\(.+\\)\\s-*$") 1 buffer))) (cl-mapcan (lambda (x) (mapcar #'list (split-string (car x) delims :omit-nulls "\\s-+"))) table))) Now we need a function that reads from the minibuffer, knows how to use these functions, and uses Org-mode's tag completion to get the sort of per-item completion you're looking for. It also needs to be aware of the capture process, so it can scan the target buffer instead of the capture buffer.
(defun my/org-capture-read-multiple (field completion &optional delims) "Read one or more items for FIELD in capture template. The COMPLETION parameter can be any sort of completion table or function valid for the second parameter of `completing-read'. It may also be the keyword :dynamic, which indicates that `my/dynamic-completion-table-by-field' should be used to generate a completion table dynamically. The optional parameter DELIMS is ignored except with the :dynamic option, in which case it is passed to `my/dynamic-completion-table-by-field' as the parameter of the same name. If this function is invoked from within an Org-mode capture process, the current buffer will be the target buffer for the capture attempt, not the buffer used to fill the capture template." (let* ((buffer (if (equal (buffer-name) "*Capture*") (org-capture-get :buffer) (current-buffer))) (completion (if (eq completion :dynamic) (my/dynamic-completion-table-by-field field delims buffer) completion))) (with-current-buffer buffer (let ((org-last-tags-completion-table completion) org-completion-use-ido) (org-completing-read-no-i (format "%s: " field) #'org-tags-completion-function))))) To use these, you'll need to modify your template. For example, you could replace the "Participants" line in your template above with
Participants: %(my/org-capture-read-multiple \"Participants\" :dynamic \"[,:]\")