Some search areas set the author of the content in userid field, some of them, there the author is not important (an activity) not, but would be nice to have a filter by author for the documents that have one.

There are two options:

• The simple one would be just an input text with a backend function to parse the input and "convert it" to a userid to apply the filter
• Use autocomplete form field type (ajax) with a complex algorithm to check permissions and retrieve only names of users you can see (you can see them all in messaging system anyway, but for what I remember that is a bug to fix, same function could be used for messaging too)