To get assistance from another player you click on them and request assistance.If they have a skill level that that is higher then yours and you need that level to be able to say *Smith* an item, then you will get their smithing level so you are able to do it. The person you are getting the assistance from gets the XP from whatever you are smithing. The person you are getting the assistance from will only be able to stand and wait while you finish using their smithing level.
Here is where you can find out all about Requesting Assistance:
http://www.runescape.com/kbase/viewarticle.ws?article_id=536
