Checking the forum of your server is a start, as there are always people advertising their linkshells (ls for short) in there.

Otherwise you can create your own by purchasing one from a linkshell vendor found in each of the three cities (they'll be marked on the map with an orb icon and have the word 'linkshell vendor' listed under the NPC's name), and then after creating it you can invite people to it freely.

It should be noted also that a lot of people regard linkshells as more than just being a chat channel, they're run almost like miniature guilds, because the fact you can be a member of multiple linkshells while being restricted to being in only one Free Company.