Simple version: Allow link sharing for forms with multiple parties

[lukas-mertens]

As already discussed in discord, I would propose a way to deal with multiple parties in a form + sharing a form via a link. This issue tracks my "simple solution" which shouldn't be much work. I will create an additional issue for the more sophisticated version which could be a great feature for the pro version.

The problem

Currently it is required to invite people to a form when multiple parties are setup in the form. Users trying to use the link get an unintuitive "not found" error.

A simple solution

Currently the invite to a multi-party-form looks like this:

I suggest to simple show the invitation form and let users "invite themselves":

Alternatively just use the existing form and provide multiple e-mail address fields. In my example I named the people "Legal Guardian" and "Authorized Supervisor" so it would look like this:

Why is this needed?

I will quote my explanation from discord:

We as an NGO need to know that the parent allows some other adult to care about their kids during our event, so that we don't have to send them home before midnight. This has to be signed by the parent and by the other adult which says "I am alright with being responsible for the child". We as an NGO need this document in the end. So the ideal process would be: We send a link to all participants with age below 18 and tell them "give this link to your parents if you want to stay longer than midnight". The parents click on the link and see our pdf in which they enter their childdrens information and who is responsible for it. They sign it. Now they enter the e-mail address of the second adult who is supposed to be responsible during our event. Now the second adult only signs the document. Our problem: We don't know who this second adult will be, as every parent might trust someone else from the participants (older brother, some friend etc.) to care about their child. The second problem: We have a lot of participants, some might want to fill out the form to stay longer that midnight, some might not. So we can't fill out this information either, we have to use the link and send it to everyone.