I'm not the best one to answer this since I mostly play single-player but I'll go with what my understanding of the current multiplayer situation is and let others correct as needed.

Meet up? Yes.
Visit the same planets? Yes.
Work together / help each other? Yes.
Trade resources? Yes.
Build a base together? Yes.

When playing as a single player (or with networking disabled) discoveries (planets, animals, etc.) are shared within the same hardware universe. PC, XBox, and PlayStation "univeses" are separate and players on one platform can not see, interact with, or view discoveries on other platforms. Player bases are visible to other players even with networking disabled as long as the player who built the base has uploaded it to the Hello Games servers. No other players are visible while in single player.

When multiplayer mode is enabled then one can see and interact with other players. Players who are on the same planet can interact even without being in the same "group" or being designated "friends." There is also a "Space Anomaly" which is sort of outside the normal rules of space and time where anyone can visit and see other players.

In multiplayer it is also possible to join the game of another player and become part of that player's "group." There are control settings to allow anyone to join, only previously designated friends, or only specifically invited friends. Joining a game saves having to journey across the galaxy (or several galaxies) to meet someone.

There are permission settings for things like "can damage" or "can add/build base parts" and the like: No one, any one, friends only, or group & friends.

So, yes, you can get together as a group of friends and explore, do stuff, build bases, etc.

One warning, though. There is a set of quests or tasks which new players (and old players starting a new game) progress through. The quests introduce aspects of the game and get players started with basic knowledge and equipment. As I understand it, it is strongly recommended that the quests be performed as a single player and not while joined in a group. I've heard that the quests for some players malfunctioned when done in multiplayer. I could be wrong, though!

Oops -- Forgot to include that although Steam and GoG players are both on PC, it is not possible for players to interact between the two. I think that bases and discoveries are shared.

There has, of course, been community interest in fixing this but the platform owners may resist making it possible.