The only person with direct commit access is the project's founder @xiaq. If you intend to make user-visible changes to Elvish's behavior (as opposed to fixing typos and obvious bugs), it is good idea to talk to him first; this will make it easier to review your changes. He should be reachable in the user group most of the time.
On the other hand, if you find it easier to express your thoughts directly in code, it is also completely fine to directly send a pull request, as long as you don't mind the risk of the PR being rejected due to lack of prior discussion.
By contributing, you agree to license your code under the same license as existing source code of Elvish. See the README at the project root for the license.