Select people in the Contacts table and enroll them in a sequence in one go.
Click "Contacts" in the sidebar.
Tick the box next to each person you want to enroll (or use the header box to select the page).
Select as many as you like — the count appears above the table.
Click "Actions".
Click "Add to sequence".
Pick the sequence. People contacted in the last 30 days are flagged so you can exclude them.