To assign an object to a group, you can simply rez it while wearing the tag of the group.
Alternatively, if the object is already rezzed, you have to:
Right click on the object and choose “Edit”.
While in edit mode, look for the “Group” detail in the “General” tab.
If the set group is wrong, click on the button with a wrench.
Select the name of the group you want to assign the object to.
Note: assigning an object to a group is different from deeding it.
If you need to deed your object, please refer to this guide.