I mainly play solo, though occasionally with a friend or two. Main difference being is that the labour is more intensive when playing solo as of course there's only you. With friends you can divide it, one builds, the other explores, and the other mines etc. No set roles but you get the idea. Combat generally is easier too, hp and enemy damage is buffed slightly to counterbalance there being more of you, but it's negligible most of the time. There's also more risk regarding playing solo too, if you die and lose your gear, there's much more effort needed to retrieve compared to doing it with friends. For example, one ally can draw aggro of the enemies away from the others, whilst the others retrieve their gear and then finish off any enemies. Basically meaning you have to be more prepared. Things like old armour and weapons, don't get rid of them. You never know what's going to happen.

And most of all, you have to employ a jack-of-all-trades-but-master-of-none playstyle regarding weapons. Of course it's not a must, but due to enemies have different resistance and damage types, it's more effective to have a variety of weapons, which with friends you can split the damage types, and due each using a specific weapon type more, their skill level becomes more proficient.