If you really, really want to use forms, you might want to look at:
1. Google Forms (
https://docs.google.com/forms). Click on the 'hamburger' top-left to get help and information. I've never used Google forms but it looks as if it's a kind of survey tool. You can view/download the responses and export them to something like Excel.
2. SurveyMonkey does the same kind of thing. It's free with limitations but you can also pay ($30) for a month if you need more respondents, questions or easy export to other Excel or other software.
3. Google Docs/Sheets. These are not interactive but multiple people can edit them. You could set up tables where the fields are self-explanatory. People can see who's already been added (and comment on these) and add new people.
Whatever 'input format' you use I suggest you think in advance how you're going to use the results. If you use some kind of family tree software (on- or offline) these often have import formats. I suggest you make sure that you will be able to convert the 'input' (probably after exporting this to something like Excel) to the 'input format' that your family tree software requires.
Personally, I would consider putting the tree up online (for example Ancestry.com) and sharing it with the people you want to contribute to it.
Mike
