Adding an outlet to unfinished basement

I want to add an outlet to my unfinished basement. Currently, there is a single 15 amp circuit in the basement that has 6 lights and 3 outlets on it. 1. Can I even add an outlet to this circuit, capacity wise? 2. If so, do I need to find the end of the circuit and go from there, or can I simply pull from the closest junction box? In other words, does it need to be in series with the rest of the items on the circuit? Thanks