For the Wave Selection an Additional Field is created using a Drop Down with the 3 options defined. 

In order to validate if that group can enter the chosen wave there are 3 parts of information we need to consider - which wave they are trying to enter, how many participants already registered for that wave, the size of the group. 

To calculate how many participants in the wave we use a DCount(), looking at just the StartWave Field.

DCount("[StartWave]="""&[StartWave]&"""")

We now need to check if this number is smaller than a certain value, since we can have 50 per wave 1 and 100 per wave 2 then there must be no more than 47/46 for wave 1, or 97/96 for wave 2, depending on whether the group registrations contains 3 or 4 members.

To be able to use the size of the group we create a separate Additional Field, called GroupSize, which is a text box. 

We include this field in our online registration form fields for Group Registrations and then set this up to use the Special Function Group Registration Count.

This value will be saved for the groups so we could also use it later to analyse how many groups of 3 / 4 we have. 

We can now use this in our Comparison, which must also consider the selected Wave. For this we use a Switch() function to select the chosen StartWave and then an If() function to return a number according to the GroupSize.

Switch([StartWave]="Wave 1";If(Val([GroupSize])=3;47;46);[StartWave]="Wave 2";If(Val([GroupSize])=3;97;96))

This will return a max number for Wave 1 or 2, but does not provide a maximum for Wave 3. We could include a further Switch and set a higher number than we expect, but in this case chose to only compare the 2 counts if the selected wave was not 3, when the wave is Wave 3, we return 1 which the validation sees as positive. 

If([StartWave]<>"Wave 3";DCount("[StartWave]="""&[StartWave]&"""")<=Switch([StartWave]="Wave 1";If(Val([GroupSize])=3;47;46);[StartWave]="Wave 2";If(Val([GroupSize])=3;97;96));1)