Minecraft Java Edition Commands – How to Add Multiple Commands to a Single Command Block

In order to use SPC you must open the chat window, then type in a command. To open the chat window, type T, then your command.

First of all, let's talk about the mistake you are making:

Your testfor command output will be permanently on, as long as there is at least one player with 30 to 49 levels, meaning the scoreboard command will only be run if there was no player in that range and then there is.

Secondly, @p[something] will always target the player nearest to the execution position that qualifies for "something". You'll need to use @a in the team join command.

That said, lucky for you the testfor is completely unnecessary^*. scoreboard can do the job perfectly fine on it's own.

/scoreboard teams join (team) @a[lm=30,l=49,team=!(team)]

The latter part of the command prevents people from being added to the same team multiple times. If there is no-one in that level range (or all of them are assigned a team already), the command will simply not add anyone to the team.

^{* In most cases, execute is strictly better than testfor}