I have a dashboard created from few reports i have saved, i want to apply this dashboard to a custom app i have created. I want few users to have default app set as the custom one i created, so whenever they login they get dashaboard as the default page.
You can set a "Default app" for the users' roles or on the users' accounts: Manager-->Access Controls-->(Roles | Users).
The users will need permissions to the dashboards/reports in Manager-->Interface-->Views. If the views run saved searches on loading, you will need to give them permissions to the saved searches in Manager-->Searches and reports.
You can also move the views and searches into the app, if desired, from those same menus.