In my Greyhawk, adventurers are less than 1% of the population, but this is partly due to the high turnover rate (i.e. death). Adventurers are uncommon, but most folk have either seen or heard of them (usually before they're never seen again). They're generally distrusted, mostly due to unconfirmed legends of adventures slaying entire villages for their "massive" hidden wealth.
There are no guilds exactly, but there are several groups of experienced adventurers (Mordenkainen's Obsidian Citadel, the Circle of Eight, and Robliar's Green Dragons). PCs are unlikely to join such groups, but may encounter some of them, possibly gaining temporary employment. It's the goal of many successful adventurers to form their own group, where they can influence events in the world from a stronghold.
Laws are applied universally, so there are few laws aimed at adventurers. Many nations have a sellsword tax, which is aimed mostly at mercenaries. There are "recovery" taxes that are aimed at adventurers, usually at 20%, designed to help the nation recover some of it's lost revenue due to the monsters/bandits/cultists/etc. Unless the adventurers are actually citizens of a location, they are MUCH less likely to be believed over the word of a citizen, so they often have to be careful about whom they deal with and how.