On the right hand side of the keyboard you will find three buttons; 'Annotate selection', 'Export annotation' and 'Clear Expression Colours'.
When you have selected some cells that will define a group (but not yet confirmed the selection), type in the text you wish to add as annotation and then press 'Annotate selection'. The annotation will be added to the current group. Do the same thing for as many groups as you wish. If you wish to export the selected groups together with their annotations, perhaps for further analysis, press export selection.

The 'Clear Expression Colours' does just as the name of the button suggests. Say you have coloured the graphs based on the expression of a gene 'x'. Then you made a selection where the gene was highly expressed and annotated it with "'x'-high" or something similar. You might then want to clear the expression colours, but using the 'Reset graph colour' button will also reset your selection unlike the 'Clear Expression Colours' button.
The 'Recolour Selection' on the Selection Tool Menu can then recolour only your selection again.