When you select many groups to bind a selectbox to users, the user are sorted group by group instead.

This means if we have those groups:

A := manuel, nicolas, sandra
B := chantal, claire, thomas

And the assigned_to is bound to A and B, then it will display:

assigned_to := manuel, nicolas, sandra, chantal, claire, thomas

instead of

assigned_to := chantal, claire, manuel, nicolas, sandra, thomas