Now that we have a new Guest List Manager, the A/B list buttons are now located on the "Responses" tab, where you will be able to see the count before the invitations are sent out, and once the responses start coming in.
The function of the A/B List feature in The Knot Guest List Manager is to help you decide who makes the *final cut* for your big celebration. A-list people are those that you are definitely going to invite. B-list people are those you're unsure about, for whatever reason. It's usually easiest to include everyone you'd like to invite on your initial guest list, and then make cuts as you determine your space and budget limitations.
Go to The Knot Guest List Manager
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See More: Guest List Manager Help