is it possible to share bandwidth b/w users in pppoe or hotspot like
if there is user1 and user 2 , and in profile i have limit on 128k/1M

now what i want to do is that , if only one user is online he can use allowed bandwidth of 128k/1M but if another user gets online via pppoe or hotspot , he shares the allowed bandwidth of 128k/1M ,
tell me for hotspot as well as pppoe , and how to do this , if possible

Use PCQ and override the simple queues created by default.
http://mum.mikrotik.com/presentations/U ... xWindt.pdf - that's for Hotspots, but should work on PPPoE as well.