Also, you might be unaware of CoCalc, which is open source https://github.com/sagemathinc/cocalc. CoCalc supports collaborative editing of many structured documents, including Jupyter notebooks, but not rich text at present.
Just to be clear, open source development of ShareLaTeX is ongoing and indeed has accelerated since Overleaf and ShareLaTeX joined forces. ShareLaTeX is split into many repos, and the one linked above is a 'coordinator' repo that doesn't get updated much. See https://github.com/sharelatex/web-sharelatex for one that is more active.
The free version of Overleaf/ShareLaTeX does have a restriction on the number of collaborators: you plus one other. Paid accounts support more collaborators [1], and you can also get more collaborators on the free account by referring others [2].
Also, you might be unaware of CoCalc, which is open source https://github.com/sagemathinc/cocalc. CoCalc supports collaborative editing of many structured documents, including Jupyter notebooks, but not rich text at present.