just use a hopper clock and do this:
scoreboard players reset @a[score_NewPlayer_min=1] NewPlayer
/scoreboard players set @a[team=] NewPlayer 1
/tp @a[score_NewPlayer=1] x y z (these are your spawn coords)
/spawnpoint @a[score_NewPlayer=1] x y z (these are your spawn coords)
/execute @a[score_NewPlayer=1] ~ ~ ~ tellraw @a {"text":"","extra":[{"text":"Welcome ","color":"gold"},{"selector":"@p[score_NewPlayer=1]","color":"blue"},{"text":" to the server !","color":"gold"}]}
/scoreboard teams join default @a[score_NewPlayer=1]
