Was discussing this earlier today, wanted to share with the community.
If you're trying to add users from a project role to Jira Service Desk's Approvers field, you can use a ScriptRunner post function to do the job!
import com.atlassian.jira.bc.projectroles.ProjectRoleService
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.util.SimpleErrorCollection
def customFieldManager = ComponentAccessor.getCustomFieldManager()
def projectRoleService = ComponentAccessor.getComponent(ProjectRoleService)
def errorCollection = new SimpleErrorCollection()
def role = projectRoleService.getProjectRoleByName("Approvers", errorCollection)
def users = projectRoleService.getProjectRoleActors(role, issue.projectObject, errorCollection)?.applicationUsers?.toList()
def multiUserPickerField = customFieldManager.getCustomFieldObjectByName("Approvers") //multiuser picker custom field
issue.setCustomFieldValue(multiUserPickerField, users)
