Retroshare not in repo?
I thought retroshare is gpled? So why is it not included in the repo?
Is it safe to add the repo for ubuntu to trisquel?
http://retroshare.sourceforge.net/downloads.html
It is not in Trisquel's repository because it is not upstream (in Ubuntu). As long as you trust whoever administrated the PPA (apparently, the developers of Retroshare), you can follow the instructions on the page you found.
And does anyone know anyone who ever met a currently active Retroshare developer?
It would be nice to know they do not belong to the dark side of the Force.
I wouldn't say it's a reliable way to figure it out, since anyone can lie.
I think it's better to check the sources, though obviously it takes specific skills and time.
The other option could be to check for alternatives already in the repo.
Maybe this is a start: https://alternativeto.net/software/retroshare/. Though I'm not sure any of these is a full alternative.