Compare 2 tables and show only "not chosen before" values

[Islom]

We have two tables including "Available groups = A" and "The student is added to groups = C" .   C table is filled with specific Lead. For example: Lead is added to the 1 and 2 groups, next time, when we add THE lead to the C table -> We SHOULD ONLY ADD THE LEAD TO THE 3 AND 4 GROUPS.

What kind of function should we use to solve  this? 

[JPAlpano]

NOT( IN([LEAD], GroupC[LEAD]) )

Or

IN([LEAD], GroupC[LEAD]=FALSE

[Steve · Accepted solution]

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@Islom wrote:

What kind of function should we use to solve  this?

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Subtract values from a list - AppSheet Help

[Islom]

Thanks! Steve It works!