Valid if dropdown as well as invalid error message

[Djigi]

Hi,

I would like to have a enumlist column [occupied] which has a dropdown menu with a list of people who are attendees, by reading two other columns and using this formula as a valid_if:

LIST([head]) + [attendees]

I would also like the [occupied] column to show an invalid message if more than 5 entries are selected.

What I tried:

AND(
LIST([head]) + [attendees],
COUNT([_THIS]) > 5
)

This of course gives the error: invalid structure: subexpressions must be Yes/No conditions.

Is there a nice way to make this possible?

Thank you in advance.