First emphasize what brings us together
Before expressing your disagreement, identify what is shared and
formulate it clearly: a point of view on the situation, a key idea, a
value to defend, un objective to reach...
For disagreement should not oppose but rather assist in finding a common position.
Formulate a hypothesis, not a truth
A differing position is better perceived and accepted when it is
advanced in a flexible way, respectful of other positions.
Inversely, affirming it as a certitude is counterproductive; it
would be better to express your idea as a hypothesis, to be investigated
and confirmed together.
Build upon the common interest
A differing position will be taken into account when it supports the common interest.
But the common interest is not always clearly set forth, and it
may be necessary to propose a formulation and to have it accepted.
Once this agreement is shared, it then becomes possible to argue
for the "dissident" position because it aims at a goal around which the
group is rallied.