I wanna be able to play with a friend and I'm not sure how to invite them

Only in dungeon possible like ancient ruins

Yes you can do party dungeon like invite your guildmates or friends in party dungeons and pvp content.
For party dungeons :
ancient ruins,primordial ruins,valor.

For pvp content : ramoness or ramoness custom, and friendly arena.

But still you can do solo in dungeons and join with bots or do random queue in pvp party.