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Go to P2P > Sites and edit the relevant site
Select Site Pages (under Building Blocks)
Edit the Registration Page
Select the Personal Details tab of the registration page
Add the Organization Handling block
Within the Organization Handling block, select which Organization Types that should be available to supporters on your peer to peer site.
Viewing and Managing Organizations for a Peer to Peer Site
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Under P2P > Sites > edit relevant site > Select Organizations (under Management)