Well it depends on the covens. If you want to join spell casters, press the Join: Accepting Applications.
or go to this link:http://www.spellsofmagic.com/coven_join.html?coven=6
The "Spell Casters" will accept you automatically, there is not priest or priestess. You will be placed automatically in "Council" position.
If you mean other covens, you do the same thing but don't follow the link, and you send the application, and you read the covens application fill out, and you send the priest or priestess or both the application form in a message. Some priests and priestess will have a coven vote..some will make the decision among themselves.