Both me and my friend installed the Destiny 2 demo (trial) from Battle.net (Blizzard) but it is not clear how can we play together, even after we completed the prologue. My friend isn't displayed in the friend list inside the game but is added in Battle.net.
Update: Apparently they have fixed something so you can now see your friends in the TAB screen and invite them to your group. Creating clan as shown below isn't now necessary. However I will leave those instructions just in case.
This wasn't quite obvious, but after some googling we found the solution.
- Complete the prologue missions(3 or 4) and get to the Farm.
- Complete a first quest on the Farm, you will be brought on a multiplayer server after this, you will see a lot of players in Farm.
- Register at https://www.bungie.net/ with your Battle.net account
- Create a clan and give your friend a link to the clan so they can enter it.
- After your friend entered your clan you can see them in the TAB screen -> Clan Players
- Invite your friend into your battle group and you can start playing together in co-op
Hope this will save time to somebody.