If you build your site on the site, then the members of your site can add and edit details in the family tree with no problem.
All you need to do is invite your family members to be members of your site!
my family members have not been able to edit on the site.
I tried to look at the privacy settings and the box to check to allow site members to "post content" for family tree page is greyed and I can't check it.
If you have a site that is published from FTB then your tree cannot be edited on line neither by you nor by family members.
It might be because of this that you see the post content part greyed.
If that's not the problem please send me your site address and I will look into it.
Yes it is possible!
Please see the follwoing post in our FAQ regarding this issue: here.