calculating s.e.m

hi ,

i am using a software that compares two groups with pairwise fixed reallocation randomisation test. my two groups are significantly different and the software gives the exact p-value.also the software tells that the second group is for example 2.2 fold the first i want to draw a graph,i need to show the standard errors as well.but the software did not tell anything about this item,how can i calculate standard error by knowing mean and number of population of each group and p-value? thanx a lot for your attention and help.