HI Seetharmi
Use Extended Notifications to send the email with the Portal URL - do a search on SCN and you will find help on this.
Use transaction SWFVISU with a Task Visualization to launch your Web Dynpro. Your web dynpro will need to accept the parameter _WI_ID and use it to read the relevant info from the work item container. It also needs to update the work item container and complete the work item. You can use the workflow APIs (function modules starting SAP_WAPI_ for this)
This a common requirement that many others have worked with. Please search on SCN for more info as you go.
Good luck
Jocelyn