Once you have created your custom block you have the ability to share them with other accounts.
Firstly, navigate to the My Blocks tab and click the tab to view all of your My Blocks. The number in brackets specifies how many My Blocks you currently have. Click the drop down menu next to the My Block you wish to share and select "Share".
A popup will appear to take you to the next stage, click the text "Share the Block" where you will be able to specify who it is shared with.
You have the option to share the block with all the Users/Clients within your Client/Group or with a specific user by simply typing in their username or the name of the Client.
Please note: the users that you can share a block with will depend on your Admin level.
After you select the Users/Clients that you want to share the blocks click Add Share.