At level 10 you can start partying in Valkurm Dunes but these days you'll level faster if you just solo to 15 or even 20. Since you're new I'd recommend that you party at 15 though to get the hang of it.

At level 18 you'll need some people to help you get the items to allow you to have a support job.

At level 30 you'll need some help to unlock an advanced job if you choose to do so.

At level 50 and every 5 levels after you'll need people to help do the limit break quests to raise your level limit.

All throughout that time you'll be needing people to level up with of course, and to do rank missions.