How do I add more members to my Apple Music family plan?

Hello, I am the administrator of my Apple Music Family plan - the plan currently has 5 members (including me) but will not allow me to add another member (the + symbol on the top right is greyed out) - I understand I should be able to have 6 family members so any ideas what I am doing wrong or how to add another member would be most welcome. Thanks

[Re-Titled by Moderator]