Family Plan

We plan to offer Family Plans and offer an add-on MarketPlace subscription too. 
We would like to offer each member of a Family Plan to upgrade to a MarketPlace subscription too.
Can this be done?
If yes, what happens when the FamilyPlan subscription is cancelled? Will the individual member retain the MarketPlace subscription?
The scenario is that a user can belong to a FamilyPlan and also have a subscription to MarketPlace. 
Please suggest a way how this can be achieved.

When a subscription is shared to another user it doesn't actually exist. It's inserted dynamically. So it has no impact on upgrades and can't be unsubscribed from. It can only be removed directly either by the user that shared it or the user it was shared to. So they should be able to have both subscriptions. You however can use CBSubs SQL Actions or CB Auto Actions to remove the shared subscription when they purchase another plan if you don't want them to have both by removing the database row for that shared subscription.

So, is the Parent (subscription owner) the only one who would be able to add a posting in MarketPlace if we put the the parent in a 'MarketPlace' usergroup with permissions to add postings, or will everyone in the ParentPlan be able to add a posting also?

Please describe what functions/features the non-existent subscribers are limited to.

Sorry, I don't understand what you're asking here. CBSubs Family Plans simply lets User A take Plan A and share it to User B. User B will have all the same benefits of Plan A that User A has. If User A cancels sharing Plan A with User B then User B will lose all of Plan A benefits. If Plan A for User A expires then the state change (via onCPayUserStateChange trigger) is also passed to User B causing any expiration behavior to apply to them as well.

In this case, will User B have identical features like User A? ...create events etc.
Will User A and User B belong to the same user group by default?
If User A upgrades to MarketPlace Plan to post ads, how will User B be affected.?

In this case, will User B have identical features like User A? ...create events etc.
Will User A and User B belong to the same user group by default?

Yes, User B will have the same features as User A.

If User A upgrades to MarketPlace Plan to post ads, how will User B be affected.?

If they upgrade from Plan A into Plan B then User B will also lose Plan A since User A (the user who shared the plan) no longer has Plan A. User A would have to share Plan B if you want User B to have Plan B features. If both plans are non-exclusive then Plan A will continue to function as normally for both users while Plan B will only function for User A since they're the only one that purchased it.

Family plans do not share parent/child subscriptions. They only share the immediate subscription and nothing more. So if you want Plan A and Plan B to be shareable you must enable both to be shareable.

If you want User A to upgrade from Plan A to Plan B AND migrate Plan A for User B to also have Plan B you need to do 2 things. First Plan A and Plan B must be shareable. Second you'll need to use CBSubs SQL Actions or CB Auto Actions with a Query Action to update the _cbsubs_family table to change what subscription is being shared since User A will have a new subscription for Plan B. If that's what you're trying to do then the below should get you started.

Integrations > SQL actions > SQL Action 1
Activation:
This would apply to all shared subscriptions from the user though so it might need more additions to the query or IF substitution for more fine grain control depending on your plan setup.

I've added a feature ticket though to look into automatic this in CBSubs Family Plans though as it's an interesting usecase allowing upgrade from one family plan to another.

forge.joomlapolis.com/issues/9763