You can add new influencers to your campaign at any time:
Total information for newly added influencers will be shown for the whole campaign period (beginning with a start date). However, historical data will start to be calculated only from the day when the influencer was added.
To delete influencer from the list just hover over settings button next to the their account:
In case if you add this influencer back to the campaign, historical data for the period their account was deleted won't be available. However, it will be restored for the previous period the account was added to the campaign.