If they have enabled it, you need to do as follows: First you must find out the plot id number, depending on what they have allowed you need to try one of three ways, the easiest being: 1: while standing in your plot do /region info. This, if allowed will tell you the plot id. You most likely won't have permission to that command. 2: you can try /srm list. This may reveal the region you own. 3: the most likely way you will have to do it can be annoying and take forever, starting with 1, you will have to enter /srm tp plot1 and go through all numbers until you get tp'd to your plot. This is how the old prison worked and had to be done. Once you find your plot id, you type the following command: /srm addmember "ign" "plotid" (without the quotations) or /srm addmember "plotid" "ign" (Can't remember which order but it's one of those two. There you have it Remember only add players you can fully trust.
