SVN/git commit access requires the creation of an account on the alioth.debian.org server. This is as simple as opening https://alioth.debian.org/ and going to the account creation pages.
Official Debian developers will probably use the same login as their Debian login, but must go through the account creation process anyway. Non-Debian developers should be aware that the login they will choose on alioth.debian.org will be followed by “-guest”. This has absolutely no meaning of any kind of second class citizens but is just an easy way for alioth.debian.org system administrators to know who is an official Debian developer.