Usually you have to email the high priest, the high priestess, or even both. There can even be two high priest or two high priestess. Some don't have any leaders at all. If that coven doesn't have a leader, then you can apply to be the leader.
Some covens need you to e-mail the high priest or high preistess, but some covens, like spell casters, you can join automatically. You also need to be council level to add spells. Spell casters also automatically places you as a council member so that you can add spells.
Always have fun in your coven, and if you need help with your coven, then you can always go to the chatter room and ask some coven members to help you. Have fun in your coven!