This is not possible in Phabricator today. While this is the situation, we recommend to keep the Joinable By project policy to All Users. See

https://www.mediawiki.org/wiki/Phabricator/Creating_and_renaming_projects#Restricting_Joinable_By

This is an option to be used mainly for Team projects where membership provides special access to features. If you want to restrict this option you need to justify this decision in your request.

Note that Phabricator allows to subscribe and watch projects only to their members. If you want more users following your activity, you should let them join the projects. If you want to define who is an official member of your team, you can do so editing the project description.

My question is why you might not allow anybody to just join the project.

@Aklapper: We normally do, but requiring people to add themselves as members of a project (which they might not feel they are if they are just casually following it) in order to even see the option of watching / subscribing to the project is not very inviting. It also makes looking at 'members' useless for getting an idea about 'who is working on this'.

I'd like to be able to give people a link to a watch dialog as an option to follow a project / tag. We actually have very targeted tags for this purpose (restbase-architecture and restbase-api) to make sure that people can subscribe only to the aspects they are interested in. We only add those tags to topics that should be of interest to the respective audiences. This has the potential to give us a very high signal/noise ratio, and partly reduce the need for parallel mailing list discussions.

All this does not work too well though if the sign-up process is a multi-step process.

Also, for comparison: On github anybody can subscribe to a repository, and this is a frequently used feature. For example, the Rust community uses an RFC repository to design the language, and invites people to follow along by subscribing to the repository. Phabricator tags could be even better than the coarse per-repository subscriptions on github if subscription / watching was easy.

In T77228#853535, @GWicke wrote:

It also makes looking at 'members' useless for getting an idea about 'who is working on this'.

In terms of our usage, this is a good point to me.

This might be very helpful, for teams to be able to limit who can move items around on their workboard, but whilst still allowing any user to watch the activity of the team, with a single click.

E.g. the Collaboration Team doesn't have any tasks of its own, but primarily exists to do management of [sprint-level and quarter-level and longterm] tasks. It is currently set as restricted access to join, so that the PM/team has control over their own schedule.
IIUC, Someone not on the team could join each of the extensions that the team deals with (Flow/Echo/Thanks/WikiLove), and I think (?) they would see exactly the same email/web updates as they would if they had joined the Collaboration Team. The only difference would be an inability to move items on the Collab Team's workboard. If so, would this be a reasonable long-term setup to leave it in?

This would also occasionally (not often) be helpful for making private Pastes, that are automatically shared with the team/colleagues who are covered by appropriate NDAs.

(Sidenote: There was also a thread discussing aspects related to this, on https://lists.wikimedia.org/pipermail/teampractices/2014-December/thread.html ("Team membership, what does it mean."))

Upstream has set a blocking task: Adding a CC to a Maniphest Task should give View rights for that user

I have mirrored the dependency here, although I'm not entirely sure that this is just as symmetric.

Naively I think the current term "Members" should just be changed to something better, but not sure if the words "Followers" (instead of "Members") and "Watchers" are separate enough when it comes to express how "loosely/closely" you watch it.

@Aklapper, to make watching a feasible 'follow this topic' mechanism we need to have a way to give people a simple link to click. A multi-step process is unlikely to get many people signed up.

@GWicke, sure, @Aklapper is just trying to find a short term improvement to clarify the current situation that we can implement here trivially. Any substantial change allowing to bypass "Join project" in order to become a watcher requires an upstream solution, probably not sort term at all.

Sadly, I don't think that renaming labels will solve much. The point of confusion is still how to watch projects when you cannot join them.

In https://secure.phabricator.com/T10180, @epriestley wrote:

Previously, you had to be a member of a project to watch the project. Now, anyone can watch a project. This doesn't let you see anything you can't otherwise see.