Create a macro to open the required form.

Go into options and select customize ribbon.

On the Main Tabs list select the group and then click on the New Group button.

Right click on the newly created tab and re name it and the sub group now you wish.

look at the Choose Commands From
combo box and select macros.

you will find your newly created macro in here , select the group you wish to add it to and click add to move it across.

Job done this should allow you to open the form from the ribbon.