Hello Veronica and welcome aboard!

You made the right choice by choosing Elgg for your social app.

Is Elgg still updated and "taken care of"?

Elgg is constantly updated and supported:
https://learn.elgg.org/en/5.x/appendix/support.html

This is how it happens on GitHub https://github.com/Elgg/Elgg/commits/master

I see a lot of posts hundreds, if not thousands, of days old

In fact, Elgg as a framework is stable and reliable, perhaps it lacks a good user interface, but there're also developers who will help you improve your app for free or for a fee.

The decrease in activity occurred due to the fact that more and more Elgg developers “grew up” and Elgg as a free and open source product rests only on enthusiasts.

There's a Telegram channel that regularly published info about all Elgg plugins and releases, as well as other useful information. Subscribe here: https://t.me/elggnews

There's chat also where you can ask questions just like in this Community: https://t.me/elggchat

unfortunately I can't seem to attach a screenshot

This community aka forum is made on the basis of Elgg version 2 and the administrators simply don't have enough time to update this site: https://elgg.org/discussion/view/3290256/elgg-community-future

That's why many things here still look strange ;)
 

is there any way I can upload an Excel-file to a group

By default, activating the "Files" plugin allows users to upload any files, including Elgg.

In the future, they can only be downloaded.

There's a 3rd-party plugin lets users preview MS Office files (doc, docx, xls, xlsx, ppt, pptx), Apple iWork pages, Adobe eps, and zip files.

I've never used it but you can download the version for ElGG 5 here: https://github.com/rohit1290/gdocs_file_previewer

 and then let the group members edit it right there?

There are no plugins for collaborative document editing for ElGG 5.

For this you need to hire a developer ¯\_(ツ)_/¯

https://elgg.org/groups/profile/75603

Hi Nikolai, and thank you for your kind and explanatory answer!

I get a followup-question on this: 

is there any way I can upload an Excel-file to a group

By default, activating the "Files" plugin allows users to upload any files, including Elgg.

In the future, they can only be downloaded.

I installed the Files plugin and managed to upload the excel-file, but if it's not editable from within the "Elgg-community" I'm planning on creating (for a non-profit organization) I will probably work with onedrive, so that will be alright, but what I don't understand above is this: "In the future, they can only be downloaded."

Is the possibility to upload files going to be removed? 

I'm thinking that then there should subsequently be no files to download either...?

Maybe I'm misunderstanding?

Thank you for all the useful links!

//Veronika

The ability to download or upload files is a default feature in Elgg.
But this option can be overridden in your custom plugin.

So :
1 - You must create a custom plugin with collaborative document editing, OneDrive integration features etc
2 - In this plugin, limit file downloads/uploads if you need this option