Disagreeing with your friends in the group is fine. Doing it without making them the enemy takes skill.

Disagreeing is the easy half. The hard half is doing it in a way that leaves a difference of opinion where it was instead of converting it into sides, because tone is what people remember long after they have forgotten what the argument was about. The skill is keeping the person next to you while the idea sits across from both of you.

Say the split out loud before you argue it: you like them and you think this particular plan is bad. Then stick to the plan and keep their name out of the criticism entirely.