Join Detection

The way I know to detect a join is by looking for people at world spawn. Of course this means that when someone dies, it will give them particles too. The way to get around this could be to set a spawn point for each player specifically when they join in. That solution will mean that they only get particles when they first join the world. If it doesn’t matter if someone dies and gets it when they spawn, I recommend the first option.

Particle Command

Play particles on the nearest player:

/execute @p ~ ~ ~ /particle minecraft:campfire_tall_smoke_particle ~ ~ ~

How to Set World Spawn?

Example setting world spawn to 0, 10, 0:

/setworldspawn 0 10 0

How to Set Player Spawn?

Example setting nearest player’s individual spawn to 10, 20, 15:

/spawnpoint @p 10 20 15