From LSWiki

The Steel Serpents

Joining

See Marshall Osaka Jizo in the Steel Serpents training hall in Corna. Marshall Osaka will also provide training for Steel Serpents. You may join, leave or rejoin at any time.

Specialty Access

Skill                  Max     Req
Combat Reflexes        +3       
Massive Blow           +3       
Pole Arm               +3         
Precision Strike       +3        
Shield                 +3       
Spear                  +3       
Subordination          +5       1
Sword                  +3       
Tactics                +5       1

Channel

Steel Serpents have the channel serpent.

Affiliation Restrictions

As guards, Steel Serpents are forbidden from being outlaws and wanderers. This excludes the Cult of Decay, the Hellwalkers, the Weapon of Vengeance, the Reapers, the Aligned, the Brotherhood of Wine and Song, the Explorers, the Hand of Eris, the Maidens of the Spear, the Travelers and the Zetesai.

Being soldiers, Aliavelyrae and Verynvelyrae will not allow Steel Serpents among their order (cannot join SS or join Aliavelyrae if already a SS).

Also, vigilantes are forbidden from joining the Serpents. This excludes Wandslingers.

Additionally, drunkards are restricted from joining, meaning members of the Brute Squad are also excluded.

Simply put you don't want to join this Assoc unless you have to, i.e. for Equites.