private _getPeoplePickerItems() {
    pnp.sp.web.siteUsers.get().then((data) =>{
      this.setState({
        DeptContact: data
      });
    });
}

<PeoplePicker
            context={this.props.context}
            titleText="People Picker"
            personSelectionLimit={3}
            groupName={''}
            showtooltip={false}
            isRequired={false}
            disabled={false}
            selectedItems={this._getPeoplePickerItems}
            showHiddenInUI={false}
            principalTypes={[PrincipalType.User]}
            resolveDelay={1000}
            />
          </div>
          </div>