Other players will naturally join your Den as it shows up in search results.

For this first version of the Den feature, if you want to invite specific people, you will need to share your Den's name with those players through some method outside the game, and then they will have to search for your Den by its name. In the future, we will improve this to make it easier to invite other players.

Note that if your Den has 30 players, it is full, and nobody else can join until someone leaves (or is kicked out).