From LSWiki

The Adventurer's guild is your default guild. When you first start, you will be a member of it.

You don't have to do anything special to leave it. In fact, if you join another guild, you'll automatically leave this guild and join the new one.

And if you leave another guild, you'll automatically be an Adventurer again.

Levelling solely as an Adventurer can actually be a viable choice, as the Adventurer's Guild offers fairly high specialty access in many but the most esoteric skills.

Being the default, there are no requirements to join.

The Adventurer's Guild gives you extra Assimilativity, however many points it takes for you to reach 0 from the negatives: a skogarhogg that starts at -10 actually has 0 while being in it, while a rhax, supposed to be at -18, also has 0. If your race has positive Assimilativity by default, then the bonus is +10, for example, a human will have 20. In the case of your race starting with 0 assim, then the bonus is +5.

Specialty Access

The following skills each have 10 access. In alphabetical order, as it's a rather long list. (Tested with a fresh Losthavener Dana, is likely accurate. shinichizio)

A: Acting, Aelvalie, Anatomy, Anglic, Angrak, Animal Lore, Aquatic Fieldcraft, Arcane Lore, Archery, Arctic Fieldcraft, Armour Lore, Armour Use, Astrology, Athleticism, Awareness, Axe

B: Balance, Blindfighting, Bludgeon, Break Fall, Breath Control, Brewing

C: Calligraphy, Cargo Handling, Carousing, Carpentry, Cartography, Carving, Centering, Channeling, Climbing, Combat Reflexes, Concentration, Cooking, Cosmology, Courage, Crossbow

D: Dagger, Dancing, Demolition, Desert Fieldcraft, Dethek, Digging, Diplomacy, Discipline, Disguise, Dodge, Drawing, Driving

E: Eavesdropping, Elude Pursuit, Equestrian, Equipment Maintenance, Etiquette

F: Farming, Fast Talk, Finance, Find Weakness, Firefighting, First Aid, Fishing, Flail, Fletching, Foraging

G: Gambling, Gem Lore

H: Haggling, Hammer, Hardiness, Heraldry, History, Horticulture

I: Imagination, Impact Absorption, Infernal Fieldcraft, Insect Lore, Instant Stand, Intelligence Analysis, Intimidation

J: Jumping, Jungle Fieldcraft

K: Killer Instinct

L: Lack of Weakness, Leadership, Leatherworking, Legend Lore, Legerdemain, Linguistics, Literacy, Livestock Breeding, Load Bearing, Lockpicking, Locksmithing, Logic, Logistics

M: Manipulation, Masonry, Massive Blow, Mathematics, Memory, Metallurgy, Mimicry, Mineral Lore, Mortuary, Mountaineering, Mounted Combat, Musical Composition

N: Nanomi, Navigation

O: Ownership

P: Pain Tolerance, Painting, Percussion Instruments, Philosophy, Physics, Picking Pockets, Piety, Plains Fieldcraft, Plant Lore, Poetry, Pole Arm, Politics, Practice, Precision Strike, Prose, Psychology

R: Recuperation, Resilience, Rhetoric, Riding, Rigging, Rune Lore, Running

S: Scholarship, Sculpture, Sewing, Shield, Shorelands Fieldcraft, Showmanship, Singing, Sling, Smithing, Somatesthesia, Spear, Spelunking, Sperethiel, Staff, Stamina, Steadiness, Stealth, Storytelling, Strategy, Streetwise, Stringed Instruments, Subordination, Suturing, Swamp Fieldcraft, Swimming, Sword

T: Tactics, Takargu, Tanning, Teaching, Telesmatic Weapon, Theology, Throwing, Torture, Tracking, Training, Traps, Tumbling

U: Unarmed Combat

V: Ventriloquism, Vocalization

W: Weapon Lore, Weaving, Wind Instruments