A friend can call you, talk on discord or social media etc. You can party up with a free trial player if they told you their in game handle. /sea all name/zone/area. Unless you are referring to "two" free trials account, than no. They can't add each other.

Is this for FF14 Night Clubbing scene? Frankly I don't understand the issues you have highlighted. Me & wife always party up with our free trial friends.