I can't join my friend in party

[MouraThat]

Yes you can , but you need to not be on the newest version of Minecraft
the /party command doesn't work on 1.15 , try and use 1.8.9 as this is the most ideal version for PVP and /party will 100% work.

 

[Subvalent]

must be pinned on the top.

 

[EA0_2_0_8_]

and if you are a party leader in a party, you need to disband first to join or else you can't join it