Admin - Manage entries based on a field type choice
It's not (yet) possible to have multiple websites with dedicated front-end and back-end views within one Craft installation (Multi-site feature). There's some users that mis-use Craft's localization feature for this, but I definitely wouldn't recommend this.

But it is actually not a problem at all to route all your subdomains to individual template folders of a single installation (→ Is it possible to route a subdomain to specific template group?). And for the backend, I'd simply create a user group for each of your events and configure the permissions accordingly (→ Assign permissions to users and groups). So user group "Summer Party" has only access to sections belonging to that sub-site. Individual users can be assigned to more than one group if necessary.

Instead of having lots of single sections ("Summer Party - Home", "Summer Party - Results", "Summer Party - Media Contacts") I would recommend to look into replacing this with a single structure section ("Summer Party - Pages") with multiple entry types. And for your media channels you could also consider not using entries at all and assign fields to your assets directly.