You can add a new record to a table if you have edit access to a base. Simply click the "+" in the bottom-left corner of the table. This will create a new empty record at the bottom of your table.
You can also insert a record by right clicking on a record and selecting "Insert record above/below" from the menu.
To add multiple records quickly, you can use the keyboard shortcut Shift + Enter. Start by clicking the primary field above the place where you want to insert new records. Then, press and hold Shift and then press Enter (holding Enter if you want to quickly add multiple new records).