On Person Search and Advanced Search, the results list has checkboxes beside the names. As soon as you check one of them (1), a tool bar will appear (2) with the option to choose a tag or committee for the selected people.
Select a tag...
… and it’s done. eAdventist lets you know what happened with each selected person. Green = “added”. Yellow = “already added before”. Red = “couldn’t add”. Hover over any of the indicators to see an explanation.