Making some groups more restrictive than others could be a compelling feature that, in some scenarios, would be very useful.

Considering that Heartbeat allows the creation of nested groups, I thought child groups could only give access to channels if the user also gets joined the parent group.

I'll try to give an example:

Imagine a Heartbeat community where some channels are only for students and others for non-students.

Accessing any channel would depend on groups, so each group gives access to one or more channels.

When somebody buys a course on my website, I'll use automation to add the user to Heartbeat and assign them a specific student group ("course A") that will give them access to a particular student's channel ("Course A" channel). The only way to access the group and the channel is to buy the course. Non-students can't access those channels.

When somebody buys a community membership, I use automation to add the user to Heartbeat, but I don't want to assign them any group. I want the groups that give access to the non-student channels to be joinable, so any member (non-student) can choose what groups to join, and as a consequence, they can filter the channels that will be visible when they log in.

However, if the channels for non-students are joinable, any student could join them too, and I don't want that.

I thought about using parent group this way:

When students buy a course, they'd be assigned to the group "Students" (parent group) and "Course A" (children group).

When non-students buy a community membership, they'd be assigned to the group "Members" (parent group). Later, they can join any "children group" that gives access to the non-students channels because they are joinable.

If a student tries to join a group that gives access to a non-students channel, it won't work because the students don't have the "Members" group assigned.

Any groups that provide access to non-students channels are children of the "Member" group, and students can't belong to this group unless they also buy the community membership.