Can't share a project or schema

I’ve created a project with 3 schemas - no problem, however, I then wanted to share it with my colleague (who created an account on mockeroo). I found the sharing option (at both the schema and project level) and added my colleague’s email address.

Problem: he could not see either the project or the schemas and when I went back to the sharing dialog there was no trace of his email address, the only one visible was mine.

any ideas?