To define the Group Structure in the designer of a treatment, decide on the participant types first. These represent user roles, which might be trustor and trustee in a trust game for example. Sticking to this example the group structure would result in one participant of each type per group, as this game is played in groups of two.
From this point you always decide on how many groups you would like to add to an experiment and not how many participants should participate. In individual experiments without any interaction the number of participants equals the number of groups created.