(*** This post was several times edited, because I noticed something wrong with the wrapping of text,
and I can't red well the text  – when I saved the post. )

Because of I've seen on your site you are engaged on several of CMS platforms, and promoting
using WP as a possible CMS environment: (I'm trying to establish such one, but that is under testing yet),but surely I was not quite specific with this issue.

It would be a great improvement in your plugin:

When assign a task to a single user that task would be allowed to assign
for several else users too simultaneously.

* Let you think, we are working in a multi-author WP environment, where more authors
could get same task assigned to work (elaborate) on  a given topics, and additionally
they are able to nicely  comment the flow of their work.
(thanks to this great new feature – recently implemented in CollabPress
…great idea :)

Technical issue about this: I was thinking a bit on how to resolve this…

You are using wp_dropdown_user() in cp-core-projects.php for generating the
current users dropdown (writing the SELECT html element into the code.) (273),
then assigning the task to the selected element in (422). (Really tricky simple –

The problem is (I can see): the wp_dropdown_user() generates  the
select element's id, and name  from the wp_dropdown_user passed parameters
which are not suited for giving how to allow select multiple and getting the
multiple selected options values.

Possible tips: I did not tried yet, the (below written are only pleriminar thoughts),
but probably would be resolved somehow to hack that select element getting
something like this

<select id="authors[]" name="authors[]" multiple="yes">

(I've stole this idea from a php manual - How do I get all the results from a multiple
select html tag   )

1. either passing an array when giving

wp_dropdown_users(
        array(
            'show_option_all' => __('Any'),
            'name' => $authors // declared before an array variable
        )

2. Or  in the further step we somehow must write on a tricky way either renaming the Select id,

and name (if the above don't work) and adding the MULTIPLE attribute for the select element
either from a javascipt or Jquery when doucument. ready 

a, either we should insert to the bottom of php a simple javascipt block
(ensuring the document had already been downloaded before referring to that
select element id

// adding attr multiple and renaming  the select element if that was originally inserted wp_dropdown_users(passing no paramaters)

document.getElementById('users').setAttribute(multiiple','yes');

document.getElementById('users').setAttribute('id','authors[]');

document.getElementById('users').setAttribute('name','authors[]');

b, jQuery(document).ready(function(){

* I'm not perfect yet with jQuery and several of their versions used  in WP

3. Further step somehow to get the selected options elements what the 
id authors[].options[i].value contain, that is your code specific part
(I did not looked into deeply yet)

Further elaborating on this, I'm using ROLE SCOPER plugin (for further defining possible
user groups, and making several restrictions) and Co-Authors Plus
(earlier mentioned on a issue). As I can see, from the code Role Scoper redefined this wp_dropdown_user() core function.  I've not tip yet how to resolve whether not only
multiple authors could be seleted but, also defined users gropus too assigning to them specific task.
But that would be also great to be resolved.

If you see fancy about this written idea, it would be welcome to see in
one of next version of your great plugin! .

(* I'm trying to play with for making a private resolution hacking your plugin at least
for the multi author selection for myself, because I'm wanting  that very improtant for my site,
BUT THAT IDEA perhaps WOULD BE ALSO IMPORTANT FOR ELSE ONES TOO,
and THAT ALSO WOULD BE MUCH MUCH MORE RELIABLE if
YOU COUD MAKE THIS CODE CHANGE! (I'm not a WP expert )

Thank you for your listening and answering to my several of issues

xy

Only a suggestion: It would be wonderful to select more users from the assign task dropdown. (later probably user groups)

Thanks, if it will be possible in a next release.

Br.

xy