Setting a password when initially accepting an invite is optional. If an invite is accepted without filling in the password field, the user will need to set a password after they log out and log back into the site.
I think that for almost all cases, setting a password when accepting an invite is more straightforward than setting the password at a later time. For that reason, I’m not sure that the password field should actually be optional. Possibly this is something that could be controlled by a site setting.
The current UI is confusing. The field is optional, but the star next to the password field name is used to mark required fields. If the field is to remain optional, the star should probably be removed from the password field on the accept-invite page.