Subscription
Pro plan subscriptions are workspace-based. This means that one subscription applies to one workspace only. Find details on Toggl pricing here.
If you have multiple workspaces and you want them all to have access to the Pro features, you need to have a separate subscription for each of them.
How to subscribe?
When your workspace is on the Free plan, your Settings - Susbcription & Billing will look like this:

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It shows you the number of users in your workspace at the moment (1) and your potential monthly fee (2) in relation to that;
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Click on 'Upgrade now' (3) to upgrade your workspace to the Pro plan;
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Choose your method of payment and hit 'continue';
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Once you've entered the required details, your subscription is set up.
I have subscribed
Your Subscription & Billing page will now look something like this:

The Billing information section holds all the important information about your subscription: current monthly charge, team size, next payment date (4), your current method of payment (5).
Read all about the Pre-payment section here.
You can find and download all your workspace invoices and payment receipts from the sidebar on the right, just below your company logo:

Canceling a subscription...
... is also done on the Subscription & Billing page. Simply click on the link (6) and follow the instructions:

You will receive a confirmation email afterwards, if you do not receive an email confirming your cancellation, please contact our support.
Note that no refunds are made for any payments that have already been made.
When you cancel your subscription, your workspace will either be:
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Put back on the free profile - if you have up to 5 users only;
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Be suspended (see below) - if you have more than 5 users in your workspace.
Suspension
There will be a box when you log in to tell you that your workspace has been suspended. This means that:
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You can no longer track time to that workspace;
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You can still view the workspace Reports and Settings.
A workspace gets suspended when it doesn't have enough balance to pay for the Pro plan. This includes:
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You have canceled your subscription, but your workspace has more than 5 users;
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Your workspace payments have failed for 7 days in a row;
To revoke the suspended state, you can either:
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Change or add payment details to make a successful payment; or
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Reduce your team size to fit the Free plan team size (max 5 users).