Suggest existing organisations / teams on signup with matching non-generic domains

### Is your feature request related to a problem? Please describe.

A common problem is that users incorrectly create a duplicate organisation because they haven't received / seen an invite from their team.

### Describe the solution you'd like.

- Allow users with non-generic emails to see existing organisations created by users with the same domain.
- Allow users to request an invite to an organisation

### Additional context

This was brought up as a way to improve time to first feature / improve onboarding