There might be some open source tools that use https://en.wikipedia.org/wiki/Shamir%27s_secret_sharing, which allows you to set a threshold for a number of people in a group with distinct keys that have to coordinate to unlock them. eg 5 shares and 3 of 5 have to combine their keys to unlock.
There is Horcrux https://github.com/jesseduffield/horcrux. One of the projects I've personally had in the back of my head is a completely self contained key system using just a self contained HTML file (so each shard is a static HTML file that can be run to read the other keys using file drag/drop).