Some NGOs allow signing up, mostly in the autumn.
Why not sign up for NGOs in the fall only?
Because you should admit people all year round.
If I would like to start helping in April, I should be able to do so.
Of course, there is some training that needs to take place – for that you could allow people joining in April, and doing the training when there’s a larger group.
Or you could make some videos for online training.
I’m all in favor of allowing signing up all year round.