Go to the NAB Group Navigator to add users to groups in the Names and Address Book (NAB) at once.
Once you load all the groups, you have 2 presentations of the data: the tree and the grid.

The tree only allows you to add users to one group at a time.

The grid allows you to add or remove users on several groups at once.

In the screenshot below, the grid data is grouped by “Member name”, multiple groups have been selected where Administrator is present.

Using the right-click menu, we can then add members to all these groups.