Nice one for getting headhunted!
The band wanted you to join & its taken them 3 years to get as tight as you say they have become, so I'm sure that they'll appreciate that it'll take you time to get up to speed & lock in with them.
Don't worry about bum notes etc. - everyone makes mistakes & worrying about it won't help you get there any quicker.
As you've said, speak to them at rehearsal & try to pin down what you should concentrate on - then at least you've got a chance.
But most of all, don't let the pressure get to you & enjoy it!
Good luck π