If you need to provide a user of another database access to your database, you can click Add Existing User from your Manage Database Users and Teams page to grant that access (either temporarily or permanently):
After clicking this, you'll see the below modal:
Just enter the individual's Redtail username in the User Name box and click add external user. This provides them with switch-in access to your database from their default database, using the "Switch Database" option accessible after clicking their name in the top right corner of the CRM:
if the user clicks Switch Database, they will then see a dialog indicating the databases that they have access to as a switch-in user:
To switch into one of your other databases, simply select it and click switch database. You'll then be working within the other database and can switch between them as you need.
Switch-in users automatically have the same admin rights and optional rights within switch-in databases as they do within their primary databases. There is not a way to assign them different admin rights than those they enjoy within their primary database.
If/when you're ready to remove their access, you can use the Disable (or the Delete) method accessible by clicking the Actions menu next to their name on the Manage Database Users and Teams page.