From https://phabricator.wikimedia.org/T194148:

Somewhat in relation to this thread, I feel that Legalpad should allow three kind of permissions: read, sign and edit. Actually, for example anyone willing to read L2 (https://phabricator.wikimedia.org/L2) to know what they'd be agreeing to in advance has to be a member of #wmf-nda-requests. Problem is that if we open L2 to anyone, then anyone can also sign it and I feel that's the reason why the document visibility is restricted, to avoid unwanted signatures. If a "read but can't do anything else" permission existed, that wouldn't be a problem and documents not containing any private information could just be displayed to the public avoiding unwanted signatures. Thanks.

As inspiration probably we can take this change:

https://phabricator.wikimedia.org/rPHABde1ec2c9e1dc

D25101: Countdown: add a capability to decide who can Create

T15208: Countdown: it's not possible to limit who can create a new Countdown