Permissions:
simplejoinleave.setmessages - players with this permission can change join and leave messages.

Syntax:
/sjlm joinmessage %player% has joined serever.
/sjlm leavemessage %player% has left serever.

To get player name use %player%
You can use Minecraft colors and formatting, for example: &c, &7, &l, etc...

Example message:
/sjlm joinmessage &3&l%player% &fhas joined server. - Player name will be in aqua and bold, and message will be white.

Customizing messages:
You can customize your join and leave messages in config.yml or directly in game with commands in the Syntax section. The plugin will automatically save new messages to config.yml and will reload config.yml so you can see your new message.