
Either through Power Pages or through the Portal Management Area, is there a way to quickly import multiple contacts from an Excel document? Or through Excel Online?
To just import Contacts, Yes.
Use Dataverses standard data import or import from Excel in a Model Driven App (e.g. the Portal/Power Pages Management App)
From a Model Driven App, assuming your Excel is formatted correctly etc, go to Contacts list view and then on the Command Bar (if your user has appropriate Security Role privileges) you have an Import from Excel option (may be under the 3 elipses).
There is also a Data Import Wizard (will also accept CSV, XML 2003 which you can save as from Excel) available under Advanced Settings - >Settings->Data Management->Imports where you will find a Data Import command bar button
You also have Data Import on the Table in the make.powerapps interface or you could also use a Dataflow (also from the make.powerapps interface)