Some of the back ends, notably
nnmaildir, now actually store the article marks with each group.
For these servers, archiving and restoring a group while preserving
marks is fairly simple.
(Preserving the group level and group parameters as well still requires ritual dancing and sacrifices to the .newsrc.eld deity though.)
To archive an entire
server, take a recursive copy of the server directory. There is no need
to shut down Gnus, so archiving may be invoked by
similar. You restore the data by restoring the directory tree, and
adding a server definition pointing to that directory in Gnus. The
Article Backlog, Asynchronous Fetching and other things
might interfere with overwriting data, so you may want to shut down Gnus
before you restore the data.