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 dialog:
Just enter the individual's Redtail username in the Current Username box and click Add to User List. This provides them with switch-in access to your database from their default database, using the "Switch Database" option in the top right corner of their CRM:
In the above example, you can see that a Redtail user with the Username "cantrell" is logged in to the "Paul Cantrell (Documentation)" database. if the user clicks Switch Database, they will then see a dialog indicating the database they are currently in along with any other databases which their username has access to:
The database you are currently logged into will display that database name in bold as well as having the circle to its left selected. To switch into one of your other databases, simply select it instead and click Switch Database. You'll then be working within the other database and can switch between them as you need.
Switch-in database users have the same rights within your database as all other users, with the exception that they are not a part of the database's Calendar. While they can create activities for others within the database, no one can assign activities to them within the database.
Additionally, 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 Deny Access method accessible by right-clicking their name on the Manage Database Users and Teams page.