For operations that act only on SharePoint groups, the User(s) tab is replaced by the Group(s) tab.
From the Available Items list, select the group(s) to which you want to add users and move them to the Selected Items list.
Note that all groups defined for the entire site collection that have at least one associated permissions level display beneath the root site. Groups with unique permissions also display beneath the site granting those permissions. Any group that does not have at least one associated permissions level for the site collection or site will not display in the list.
TIP: If your Available Items list is particularly long, you may find it useful to narrow the scope by searching based on Group name, site Name, and/or site URL) and searching for the group. For example, if you want to narrow your scope to all group owners, enter "owner" in the Group field.