Joining Accounts (on paid plans)
Just consider the scenario: initially the members of a family create free accounts, one for each. After some time, they decide to upgrade it to one of the existing paid plans.
However, it seems that right now they could either upgrade just one account and create new addresses on it (which is cumbersome) or pay for an upgrade on each one (which is nonsense).
Simple example here: me and my wife are beginning to use protonmail, each one with its own free account, and we foresee the possibility to upgrade for a paid version in a near future. However, we won't want have to create a new address to one of us or to pay for 5 GB for each one of us (5 GB will be enough space for both of us).
In such scenarios, it will be a nice feature to offer. Is it feasible however?

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yssd commented
I have requested this feature before. same case for me. nothing done so far.