Count the number of shared names by other groups

Dear all,

I made an example here for my problem. There are 4 groups in columns A:D, which are named as G_1, G_2, G_3, G_4.

In cell G1, I use the formula below to calculate the number of names in group G_1 that are also exist in group G_2:

={SUM(IF(INDIRECT(F1)="","",IF(COUNTIF(INDIRECT(F2),INDIRECT(F1))=0,0,1)))}

Now my question is: how to find the number of names that are shared by ALL OTHER groups (i.e. not just a group)?