GroupSeq: A GUI-based program to compute probabilities regarding group sequential designs. The package provides a Tcl/Tk-based graphical user interface. It enables the construction of group sequential designs based on normally distributed test statistics, and particularly to compute critical boundaries, power, drift, and confidence intervals for such designs. All computations are done via the alpha spending approach by Lan-DeMets. Various alpha spending functions are made available to the user for this purpose.