It is the volunteer's responsibility to obtain any necessary visas and permits before making travel arrangements. WWOOF Greece cannot assist with this.
WWOOF is not about paid working. Therefore WWOOF organisations (such as WWOOF Greece) and WWOOF Hosts CANNOT help you obtain visas, work permits or letters of recommendation / invitation.  It is up to you to find out what you have to do in order to enter Greece for your visit / travels.

There is a possibility that you will be held at the point of entry and returned to your home country if you arrive without the correct documentation.

All citizens of Shengen countries do not need a Visa (European Union countries are members of Shengen).

Visa information of the EU Commission for non-EU National.

(If you are a student, investigate reciprocal temporary work permits.)