Is BBS+ 1. a multi-messages signing protocol or 2. a group signature signed by a member of a group anonymously?
I've always used it as #1. Hyperledger Ursa has an implementation in Rust
(see https://github.com/hyperledger/ursa/tree/master/libzmix/bbs).
However, it is a type of group signature which allows the type of signing of multiple messages.
When someone says to me group signature I immediately think your #2. If we look at a paper written by David Chaum (https://chaum.com/publications/Group_Signatures.pdf), He mentions
group signatures are a &quot;generalization&quot; of the credential/ membership authentication schemes, in which one person proves that he belongs to a certain group.
So in effect, the proof being made is proving the messages belong to the signature and the signature allows for a proof of knowledge to be made.
It's a different use of a crypto primitive.
Hope this helps