Put it around single quotes. This should work.
git add '*s.rb'
UPDATE
After more testing, I find out that I can even do git add *s.rb without quotes. This might just work for me with my git version 1.7.10.4 (Mac OSX Lion, homebrew). My explanation is that if shell wildcards expansion doesn’t match any file, it’d supply the original unexpanded argument ‘*s.rb’ to git add.
sh-3.2$ git status
#
# modified: config/routes.rb
# modified: spec/models/products_spec.rb
#
Now I do git add without quotes.
sh-3.2$ git add *s.rb
Surprisingly it works!
sh-3.2$ git status
# On branch master
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# modified: config/routes.rb
#
# Changes not staged for commit:
# (use "git add <file>..." to update what will be committed)
# (use "git checkout -- <file>..." to discard changes in working directory)
#
# modified: spec/models/products_spec.rb
#
If wildcard pattern doesn’t match any file, git will give out this error.
sh-3.2$ git add *x.rb
fatal: pathspec '*x.rb' did not match any files