You may join a PTA and become a member by paying dues for that membership anytime during the calendar year. After March 31st, people may want to join for a variety of reasons - to run for office, to vote in an election or even to qualify for scholarships or awards. After the units have returned their unused membership cards to the state PTA they can request an additional card or however many they need. When the unit membership chair or president becomes aware that you wish to become a member, all they have to do is contact the region membership chair to acquire more cards.
The invitation to join PTA is a standing invitation and has no time table - it should be offered year-round.