Generic login link for client login, + app access for client

I want to put a generic BugSmash login link inside my client portal for easy access for all my long term retainer clients.

I want to invite them to a project which will act as a workspace, no need for a password, they will just need to enter their email to access the correct project without accessing other projects.

As of now I send them a the unique link for their project but they constantly lose it and email me to ask for the link when they want to request an update.

Another nice touch would be if I could send them a link to the bugsmash mobile app, they sign in with their invite email and access their project (only theirs) anytime, right from their mobile phone.