You can use “a” (bbdb-add-or-remove-mail-alias) in BBDB buffers to add a mail alias to the current entry, or “* a” to add a mail alias to all displayed entries. I use mail aliases to tag or categorize my contacts (example: emacs, writing, etc.). The following functions can make it easy for you to filter displayed records using a combination of keywords:

Here’s the code:

(defun sacha/bbdb-filter-by-alias-match-all (query-aliases record-aliases)
  "Return non-nil if all QUERY-ALIASES are in RECORD-ALIASES."
  (let ((result t))
    (while query-aliases
      (unless (member (car query-aliases) record-aliases)
        (setq query-aliases nil
              result nil))
      (setq query-aliases (cdr query-aliases)))
    result))

(defun sacha/bbdb-filter-by-alias-match-any (query-aliases record-aliases)
  "Return non-nil if any in QUERY-ALIASES can be found in RECORD-ALIASES."
  (let (result)
    (while query-aliases
      (when (member (car query-aliases) record-aliases)
        (setq query-aliases nil
              result t))
      (setq query-aliases (cdr query-aliases)))
    result))

;; Moved this to a convenience function so that we don't
;; have to deal with invert and property splitting.
(defun sacha/bbdb-filter-by-alias (bbdb-records
                                   alias-filter-function
                                   query
                                   &optional invert)
  "Return only the BBDB-RECORDS that match ALIAS-FILTER-FUNCTION.
ALIAS-FILTER-FUNCTION should accept two arguments:
 - QUERY, a list of keywords to search for
 - aliases, a list of keywords from the record
If INVERT is non-nil, return only the records that do
not match."
  (delq nil
        (mapcar
         (lambda (rec)
           (if (funcall alias-filter-function
                        query
                        (split-string
                         (or (bbdb-record-getprop
                              (if (vectorp rec)
                                  rec
                                (car rec))
                              'mail-alias) "")
                         "[ \n\t,]+"))
               (when (null invert) rec)
             (when invert rec)))
         bbdb-records)))

;; Splitting this into two functions because of interactive calling.
(defun sacha/bbdb-filter-displayed-records-by-alias (query &optional any)
  "Display only records whose mail-aliases match QUERY.
If ANY is non-nil, match if any of the keywords in QUERY are
present.
See also `sacha/bbdb-omit-displayed-records-by-alias'."
  (interactive (list
                (let ((crm-separator " "))
                  (completing-read-multiple
                   "Mail aliases: "
                   (bbdb-get-mail-aliases)))
                current-prefix-arg))
  (when (stringp query)
    (setq query (split-string query "[ \n\t,]+")))
  (bbdb-display-records
   (sacha/bbdb-filter-by-alias
    (or bbdb-records (bbdb-records))
    (if any
        'sacha/bbdb-filter-by-alias-match-any
      'sacha/bbdb-filter-by-alias-match-all)
    query)))

;; Splitting this into two functions because of interactive calling.
(defun sacha/bbdb-omit-displayed-records-by-alias (query &optional any)
  "Display only records whose mail-aliases do not match QUERY.
If ANY is non-nil, match if any of the keywords in QUERY are
present.

See also `sacha/bbdb-filter-displayed-records-by-alias'."
  (interactive (list
                (let ((crm-separator " "))
                  (completing-read-multiple
                   "Mail aliases: "
                   (bbdb-get-mail-aliases))
                  current-prefix-arg)))
  (when (stringp query)
    (setq query (split-string query "[ \n\t,]+")))
  (bbdb-display-records
   (sacha/bbdb-filter-by-alias
    (or bbdb-records (bbdb-records))
    (if any
        'sacha/bbdb-filter-by-alias-match-any
      'sacha/bbdb-filter-by-alias-match-all)
    query
    t)))

This will be part of my book, Wicked Cool Emacs. Looking forward to putting it together!