You can add multiplets by picking and selecting the peaks that make up a multiplet. Then either right-click on one of the peaks and select New Mutliplet or use the keyboard shortcut AM. Multiplets are shown by a cross with a circle round it and lines linking it to all constituent peaks. A multiplet's chemical shifts are simply the averages of the chemical shifts of its constituent peaks. Multiplets can be added to any 1D or ND spectrum.
You can place marks through one or more selected multiplets either by right-clicking on one and selecting Mark Multiplets or with the shortcut UM. The shortcuts UX, UY, UZ and UW will place marks only along the X, Y, Z(1) and Z2 axes of the selected multiplets.
Multiplet lists are nested below spectra along with Peak Lists. Open a multiplet list by dragging it into the drop area. On the left-hand side you will see a list of the multiplets and on the right a list of the peaks making up the selected multiplet(s). You can select multiplets and peaks as normal. Selecting them in the table also selects them in your spectra and you can double-click on any of them to navigate to that position in your spectra.
By double-clicking on a multiplet in a spectrum (or several if also holding down the Ctrl/Cmd key) you will select all the constituent peaks. This can be useful if you want to assign/label all peaks the same way in one go in the Peak Assigner. Note that if the Peak Assigner tells you that the peaks diverge, you may have to increase your Assignment Tolerances in the Dimensions tab of your Spectrum Properties dialog (open this by double-clicking on the spectrum in the sidebar).