Hi All I have a portal page having more than one channel. Can I decide

It depends on what you want to display. Not on the channel itself but you can display the content only if this user is viewing the dashboard. This only works for web content with dynamic data.

If you want to show or hide a channel at runtime, you need to know the group prior-hand. This is what you will set as the Channel target. If it's the data within the channel, you have some degree of control over it by using the process-instance security. For e.g. a single report can display different data based on the security of the user. Same applies to KB or Folders. As @Stefan had stated, Web content with Dynamic data channel allows you to show info by putting Expressions based on role. This is an ideal candidate for use cases such as yours.
With the above said catering for most of the requirement, what else do you need to do at Runtime that cannot be categorized within these categories?