In a Sharebox+ subscription you can add as many members as you wish.
Android:
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Step by Step:
Go to the menu
Press Sharebox+ (manage subscriptions)
Select your subscription.
Select pane "Members"
Press the "+" button
Add the member from your contact book or type in the number you want (remember the country code if it is a foreign number you are sharing with)
Repeat for each new member
At the same place, you can also remove a member by pressing the appropriate member and select "Remove."
iOS:
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Step by Step:
Go to the menu (i).
Press manage subscriptions.
Select your subscription.
Press the plus button.
Select "From Address book" or "Enter Mobile Number".
If "Address book" select "Recipient" from your phone list.
If "Enter mobile number" "Enter mobile" and press OK.
Repeat it for each new recipient.