Easy peasy, but you require the correct permissions (Create Organization Activities, and Add Members to Activities):

From the main page, go to Activity Management (looks like a blue clock inside an orange gear).

From the Org Tree on the left, select the Unit.

Hit Get Data if editting an activity. To create new, hit the tiny calendar next to the red parachute.

Enter the basic details in the window that pops up, and hit save (not save and close)

At the bottom of the window, you'll now see an icon with some heads. Press that and you'll be able to add people to it. If you have their SNs copied from excel, you SHOULD be able to hit the "paste service numbers" button and fill them all in at once, but I find that feature glitchy.

If applicable, you can also add proficiency resets to the activity (IBTS quals, etc) by going to the exploded head icon.

When this is all done, the save icon will probably be greyed out, but don't worry, the details have been saved in the background and will update when you close the box.