If a participant declined an earlier invite but you've since agreed on a time that works, you can re-invite them directly from your participant list.
First, make sure the agreed time exists as a session in your Calendar tab. If it doesn't, add it before re-inviting, since you can only invite participants to times that are in the calendar.
Then:
Click Filter, select Name, and type the participant's name.
When their row appears, click the dropdown button next to their status.
Select Invite and choose the time you agreed on.
The participant gets a fresh invite to accept, and once they do, the session is confirmed as normal.