Can players assist on neighbors' homesteads?

Getting there runs through the social systems rather than a public map. You reach someone's personal home plot on a co-op invite, and you can pull friends into your own world the same way, so visiting is reciprocal: you invite them to your Homeland, they invite you to theirs. Nobody wanders onto your plot uninvited.

Once you are standing on a neighbor's land you are not just a spectator. The housing system lets you work their plot, so farming is something you can do on ground that is not yours. That makes a friend's homestead a second place to put in work rather than a place to admire the decorating.

The practical read: keep a small group of players you actually swap invites with. A homestead you can visit is a homestead you can farm, and a friend list that trades invites regularly is worth more than one that does not.

Not settled: What assisting pays the helper, and whether farming a neighbor's plot draws from their harvest or adds to yours, is not settled.