I don't know as they'll assume you're already members so much as eagerly pass word to let you know that you and Lee could demand to be let in. They'll then ASSUME you're members and if you deny it, they'll wonder what in Merlin's name is wrong with you.

So perhaps you might mull over how you'll deal with THAT dilemma.

It might be better to suggest that they can join once they've completed their NEWTs. Then issue the invitations as appropriate when they come of age.