There are many benefits to joining a Free Company, depending on the kind of FC that you join. The ones sending you random invites are just looking for people to add to their roster, so you're justified in finding it as almost spam. While I can't speak to the quality of any of them and whether or not you'd find a good home off a random invite, the perks of being in a Free Company can be hit or miss. If you join one with lots of people leveling, it's likely they're going to have the experience point boost Action executed, so you would gain an extra 10% experience points. There are a lot of other Actions they could have going, like increased food duration, gear durability, or teleport cost reduction. These are quality of life things, though, and you aren't necessarily missing out by not being in one. Everyone else gave good advice on how to squeeze out extra experience during lower levels.