IIRC the very beginning tutorial section of the game is completely a walled garden and you can’t see any other IRL players until you reach a certain point in the MSQ (Main Scenario Quest).

At level 15, you will unlock your first 4-person dungeon, which you and up to three other people can queue for (if you have 4 total players with the correct party composition, you don’t have to wait for matchmaking). If it is just two of you, you would queue together and wait for the party to fill.

It is around this time that MSQ unlocks your ability to fly to other main cities and travel more freely.

You can then be in the overworld doing FATEs and killing mobs together with a partner or group, but MSQ is largely a solo affair in terms of cutscenes and things that will separate you, but you can play together outside of those, and group every time a 4-, 8-, or 24-man instance is unlocked.

I will say that at least one of you should probably have or know a paid player account, as you cannot invite each other to party as two free trial accounts. It is one of a few downsides to the free trial that really gives you more free time than necessary to fall in love with the gameplay and characters.

I hope I have all the information correct, it has been a couple years since I started but hopefully I can answer any other questions you may have.