You can move one or more blocks from the canvas into an existing group. All existing wires are retained.
You can also drag a block from the palette into an existing group. See
Adding a block.
You can only move/drag blocks into a group when its contents are visible, that is, when the group is currently expanded. See also
Collapsing and expanding a group.
Or select the group into which you want to move the blocks. Then click
in the toolbar of the model editor, or press Ctrl+G.