>>Can you only do this with a paid account? I cant see the option...<<
The tree program is FREE, you don't need to be a paying subscriber to use it.
Two non-paying subscribers can share their trees with one another by inviting one another to their trees.
Non-paying subscribers cannot view ACOM public member trees on the ACOM site, however (as frost described), you can go to sister site, mundia.com and view ACOM public member trees, for free.