All apps — including the SendGrid app — are based on a specific view.
So if you’d just like to send emails to selected records, you would need to create a view that only shows those particular records.
For example, one very easy way of doing this would be to make a view that only shows you records that have a checkbox field checked. So, whenever you check a checkbox for a particular record, it will show up in that view. When you want to remove the record from that view, just uncheck its checkbox.
Alternatively, if you don’t mind the emails being sent from Airtable (instead of your own SendGrid email account), you could use the new automations feature to send automated emails when a record matches a certain condition. This will only send one email at a time, because it only works on one record at a time. (In other words, it doesn’t depend on views.)