Projects can only be shared with people if you know their exact email address

[TjerkNan]

Is your feature request related to a problem? Please describe.
It should not be possible to share a project with people you don't know about.

Describe the solution you'd like
If people want to share a project with somebody else, they should only be able to share it with somebody who:

1. Is registered on the platform as a researcher/creator (already true)
2. you know the exact email address for

[mellelieuwes]

[TjerkNan]

Confirmed this works