PowerPivot and PowerView for most users. Sql Server Management Studio (SSMS) can also be used to do cube browsing although a power user can't do much else with it.

I would probably limit SSMS use to developers and admins.

As for licensing, your best bet is to discuss that with a licensing reseller. My understanding is that anyone connecting to a sql database must either 1) have a client access license or 2) be connecting to a 'per processor' unlimited-licensed sql database. This allows unlimited client connections.

Both licensing models give a 'client' access to Sql Server Reporting Services (SSRS). SSRS also offers an interface for building mdx almost identical to PowerPivot. This can be accomplished from within Visual Studio (primarily a developer tool). You can also enable SSRS's 'Report Builder' tool which is more appropriate for your cube-browsing, report building power users.