Q: Share calendar account with Exchange calendar service

Hi I have issues trying to share my exchange calendar account with a fellow co-worker.

I tried the following steps

1. Choose Calendar > Preferences, then click Accounts.
2. Select the calendar account.
3. Click Delegation, click Edit, then click the Add button .

However, when I click on edit, I am unable to click the Add button +

Anyone knows what is the issue and how I could try to resolve it? Thanks in advance.