Ability to @mention and invite non-users of site using email address

I'm trying to use the BitBucket (BB) issues feature to track bugs and new features for my projects. I'm interfacing with a lot of non-programmers right now and they don't have a BB account. I'd like to @mention them using their email address instead of requiring them to create an account because it is hard to motivate everyone I interact with to sign up for BB.

From BB's perspective, supporting this and allowing non-users to view issues might be a gentler way for BB to on-board users.

I realize this sort of mechanism would create a spam problem.

Apologies if this is a duplicate, but I couldn't find the dup' if it exists.