A Star Group is an ensemble of stars : the variable star to be studied, comparison stars to determine the magnitude, and eventually other variable stars in its surroundings. You can see this as a template that will be laid over the photos that one will do photometry on.
Once a Star Group has been choose, the detail of the Star Group can be inspected with the button "Detail Star Group" from the dashboard. 
Then, you arrive at the screen to inspect and to work on the Star Group :
Looks a bit complicated, lets inspect its different components.
Good to Know : if you tick on the name field (the header field) of a column, the table is sorted by that column. This is valid for all tables of AstroPie
The columns :
Name : name of the star
Comp Star : type of star.
V : Variable
R : Reference star, used to calculate the magnitude of the variable stars
C : Check star
G : star created from Gaia Catalog
U : Star created from UCAC4 Catalog
any other letter : same as "V"
ra : Right Ascension
dec : Declination of star
mag : magnitude indicator used by AAVSO
V : magnitude in V
B-V : Blue minus V magnitude
radius : radius of the area that will be around the star for the photometry. In fact, this parameter is not very important any more, as the FWHM mode for doing photometry is working more efficient
Radius_in : inner radius of the ring aroud the star for determining background
Radius_out : outer radius of the ring around the star for determining background
Ensemble : AAVSO identification of the ensemble of the Star Group
Type Variable : what kind of variable
Periodicity : Periodicity of the variable
Magnitude Range
Auid : unique identification in AAVSO Catalog
Ok : if the graph of the star must be shown by default on Photometry screen
Sky Coord : should not be shown...
Catalog : where the data are coming from
Number of Hits : number of photos where the star has been found in last photometry of this Star Group
Magn. at Max : highest magnitude the star can reach
Ang. Dist from Star : angular distance the star is away from the main star of the Star Group
B : magnitude in blue
Rc : magnitude in Red
Ic : magnitude in Infrared
U : magnitude in UltraViolet
St. Dev. : standard deviation of the magnitude of the sar during last photometry of this Star Group. Only interesting when doing Time Series. Used to evaluate quality of Comparaison Stars
Good to Know : you can change the values, just by overtyping, It can be done also by using the button "Show Detail".
Good to Know 2 : a line can be deleted by using the button "Show Detail".
Good to Know 3 : the radius, radius-in and radius-out column determine the size of the aperture and the ring around the star to do photometry on.
it can be changed star by star. Do not forget to save your changes with the "Save Star Group in Coordinates file".
Their default values are in the AstroPie.ini file :
radius=10
radius_in=10
radius_out=17
But, in AstroPie.ini there is another important parameter :
FWHM=1.5
FWHM_prefer=True
If FWHM_prefer = True, then the radius is ignored, and replaced by the parameter FWHM times the measured FWHM of the star (which stands for the size of the star on the photo). This gives the best results for the photometry.
button : all ok = on, and all ok = off
These two buttons are to place "on" or "off" the values in the "ok" column.
That column decides if on the Photometry page the graph of the star will be shown, or not.
Photometry is always done on all the stars.
button : Save Star Group in Coordinates File
Each star group has its own coordinates files, located in the Coordinates directory (wow).
Without using this button to save the Star Group, all your changes will be lost.
button : Get AAVSO Comparaison stars
This button searches the star in the AAVSO VSX catalog (internet connection needed).
It's case sensitive, thus Canes Venatici is CVn, and not Cvn, Ursa Major is UMa and not Uma.
Not Good to Know : the files with these data are all in the Coordinates directory. If you want, you can change them with Notepad or even Excel. But, don't do that.
This will be the result of the search :
Sometimes there will be comparaison stars that do not interest you, they can be deleted via the button "Show Detail". Which will be explained later.
Button : Get Nearby Variables from AAVSO VSX
Once you have the comparaison stars, you can import in the Star Group also other variable stars from the AAVSO VSX star catalog
The parameters "Magnitude at maximum not fainter than", and Max. Angular Distance (in min.)" do limit the stars that will be imported.
This is the result :
Again, sometimes there will be stars that do not interest you, they can be deleted via the button "Show Detail". Which will be explained later.
Button : Get Nearby Variables from UCAC4 or GAIADR3
For UCAC4 the catalog "I/322A/out" from Vizier database is interrogated (internet connection needed)
For GaiaDR3 the catalog "I/350/gaiaedr3" from Vizier is interrogated (internet connection needed)
The results are added to your star group, here from catalogs UCAC4 and GAIA.
In the Comparison column the UCAC4 stars are indicated as "U", the Gaia stars as "G". That means the same as "V", variable star. Can be changed to R or C or...
Good to Know 1 : if you've made a mess of your Star Group, but not saved it with the button "'Save Star Group In Coordinates File", you can get your older version by leaving this screen and via the button "Select Star Group To Change it" return here
Good to Know 2 : if you've made a mess of your Star Group, and you have saved your mess, you can start over again with the button "Get AAVO Comparaison Stars".
Good to Know 3 : if you do not save your changes, they will not be saved (and thus they will be lost, aja).
Good to Know 4 : If your are fine with your Star Group, save it with the button "Save Star Group In Coordinates File", it will become visible in the Star Group screen so that you can choose it.
Button : 1 Star
To create a single new star in the Star Group.
Still to be documented