How many players can join?
The Demo supports 1–8 players, including yourself, as well as single-player play.
The play-selection menu offers PLAY OFFLINE, HOST A GAME and JOIN A GAME. Steam friend invitations and joining were added in the September 16 Demo update.
How shared quest progress works
The October 31 update v0.1.036.28825 made most quests world-shared, according to the official patch notes. The announcement does not say that every quest is shared, or specify how distance, joining late or individual objectives are handled. Check your main-menu version before comparing results.
Earlier developer advice from October 23 said both players should have the quest and be together when its completion step was reached. Players also reported differences between objectives. That advice and those reports predate v0.1.036.28825; they are not a complete set of rules for the updated quest system.
Is experience shared?
Do not assume it is. In the October 23 developer discussion, shared experience was treated as a feature to look into, with its rules still to be decided. Quest completion and experience gains are different systems.
Can you browse strangers’ servers?
The October 30 developer reply said stranger-server discovery was planned but not for the Early Access launch. Playing normally with friends is separate from that discovery feature.