Exactly...Another thing to do, especially in a new situation...is to casually approach the main offender after a rehearsal. Let them know how lost you are on a particular song(pull the thumb), and might they get together off-line to work out a particular passage?
This avoids embarrassment of the offender...putting them on the defensive... Usually players in a one-on-one situation will hear their mistake. Make them feel like they played a part in the correction and reworking of the toon. It creates a trust level that you are there to create and inspire.
When the next rehearsal happens, give them full credit for coming up with a cool new arraingement. Let everyone know that you are willing to work out any kinks or modifications to make the music better.
It's amazing what you can get accomplished if you are willing to let someone else take the credit. They in their heart will know. And the next constructive point will be better heard.
It's like vocalists working out harmonies offline in a separate session.
Suggest (the above approach is surefire) that you two get together casually to work out some guitar parts... Even beyond chords... weaving like Ron & Keef, or whoever is relevant to you.
It's usually the "in front of others" thing that falls flat, especially in a new group.