[manutrevor]

I am getting married next month, and we are looking for an app that allows multiple people to upload photos, preferably by hashtagging. we don't want everyone to have to download an app. Ant suggestions?

[ManiacJoe]

Seems like Instagram would be an obvious choice.
Whatever you decide on, everyone you want to use it will have to download it. That is just the way the system works.

[hallux]

Google Photos can do it also, and should be pre-installed on any Google device. You can create a shared album that I believe you can then invite people to and they can upload to it. A similar use case was actually used to demo the feature during an I/O event.

https://support.google.com/photos/answer/6131416

Most people will likely have a Google account already, not everyone is on Instagram.