Sending and accepting an invite
Go to Settings on the page you want to share, find Add Agency to Manage the Page, and enter the email address of their Klick.ing account. It checks eligibility as you type, so you'll know before you submit whether that account can be added.
You'll be asked to re-enter your own password to confirm. That's deliberate: granting someone edit access to your page is the kind of action that shouldn't be possible from a browser tab you left open on a shared computer.
The invite then sits as pending until they respond. On their side it appears under More > Account, where they can accept or decline. Once accepted, your page shows up in their "Pages you have access to" list and they can switch to it like any of their own.
If they decline, there's a short cooling-off period of three days before you can invite that same account to that same page again. You can cancel a pending invite yourself at any time before they respond.