Can I add objects to an existing group?

In order to add objects to a group, you must be a Site Admin with write access. 

From the Configure Site page, you will see your site with a carrot to expand on the left. You can expand your site to see BACnet devices on your network. Continue expanding devices until you find the objects you want to add to a group.

Check the boxes next to the objects and then check the box next to the group you want them added to in the Groups List on the right. Click Add to Group and the selected objects will be added to the selected group.