Inviting a Client to a meeting ? Seems not to allow

Inviting a Client to a meeting ? Seems not to allow

 In projects, editing a meeting, I'm trying to add a client to a meeting. But the client is not on the drop down list of possible invitees. The employees   contractors are listed, but not the client tyope. Any ideas ?

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Info:

Using the FREE simple version for trying things out.

Put in a example "Client", clearly shows on list of users, but not on list of possible invitees.