This is a Rust project that exports a C FFI which is documented in interface.h. It allows for N of N multisignature accounts with Nano.
An overview of the MuSig algorithm can be found here: https://blockstream.com/2018/01/23/musig-key-aggregation-schnorr-signatures.html and the paper can be found here: https://eprint.iacr.org/2018/068
This library does require the R value commitment (the 3 round version), because unlike the linked overview says, the scheme without it has been proven insecure. See https://medium.com/blockstream/insecure-shortcuts-in-musig-2ad0d38a97da for more details.