The MSP portal allows MSP organizations to link organizations you managed to your place. Here is how it works
Go to Settings > MSP Portal
Click "Link Organization"
Type in the Organization name and the admin email of that organization
The admin will receive a 6-digit code in their inbox
Use the 6-digit code
Once it is linked, you can click the "Login as user" icon on the right to jump into the organization as the admin account you just linked to.