The same way you do with a normal account /s for just general or /p for party. It should be noted however that on trial accounts, you can not use the public channels such as /trade and /general.
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A trial account itself is not the issue, it's the installed program (client). Once you have that installed, and you have made an account with a private server, it will work regardless of you also having a trial account at Battle.net.
Yes, you can use any addons that a full account can use. There is no limit.
No, the WoW free trial only includes the original game. There are no expansions in the free trial. Also while playing a free trial account you are not able to receive mail, whisper other players (unless they have you as one of their friends), or talk in channels (i.e. Trade, General)
Normally World of Warcraft offers a ten day trial which requires you to create a Trial account and download the game client, both can be done from either WoW-Europe.com or WorldofWarcraft.com for US users.
You can create a Trial Account for World of Warcraft by downloading the trial. If you later wish to upgrade to a full account you will have to buy the game and completely download or install it but you will be able to retain any account information or characters you made during the Trial Account. It is advisable to find a friend who plays World of Warcraft and creating an account (trial or not) through the Recruit-a-Friend option. If you should ever decide to make a full account, you'll get all of the rewards offered from Recruit-a-Friend and make playing the game more fun for you since you'll be playing with someone you know.