In my 2.5 installation, I created a page for wiki administrators that listed online users, using the following macro:

{userlister:online=true}

But, when I upgraded to version 2.7, I get the following error message on my wiki page:
Please specify a value for 'group' or 'groups'.

The wiki notation help (when in the Wiki Markup editor) does not list "group" or "groups" as a required parameter. I looked in the documentation, and the documentation does now list it as a required parameter.

When I write the macro as follows:

{userlister:groups=*|online=true}

I get a list of all users in the system (both online and offline).