in start workflow i need to only get users that are site members
I think that a good option could be create a custom form control based on the out of the box authority one.
In this new control you can configure the Alfresco.ObjectFinder to use your owns backend services to obtain the list of user.
You can use this new control in the definition of the start form of your workflow. You can use a control param to indicate the site in which get the users.
Retrieving data ...