Hello and welcome. I know you can get guest passes for Palmetto Dunes. I think a 1-2 day pass is free and there are nominal fees for 3 - 7 day passes. Depending on where you want to go in Palmetto Dunes, you may not need a pass at all. As for the other communities, it varies, but most allow some type of temporary guest access. Below is a link to another site that has a rundown on all the plantations and what's inside.
There are year-round Hilton Head locals, like Dave, who are a wealth of information about the island. If you have any other questions, feel free to ask.