The Weblogin snippet has basic usage examples in the top of the snippet code, and the default template is found in the assets/snippets/weblogin/websignup.inc.php, the last section of the file.
You can edit that file (make a backup copy just in case), or better take that default form as an example and create either a chunk or a document with your customized form and pass that value to the snippet to use.
If you want to add the new user to a specific group, pass that to the snippet like so:
Use a comma seperated list of the group names if adding to multiple groups:
basic no frills: [[WebSignup]] (uses the default template and adds to no groups)
chunk ex: [[WebSignup? &tpl=`CustomSFChunk` &groups=`NewsReaders,WebUsers`]] (CustomSFChunk would be your chunk name)
doc ex: [[WebSignup? &tpl=`25` &groups=`NewsReaders`]] (25 would be the document id, note that only the document content is used)
Hope this helps