The Assassin's Guild was an organization whose members were professional killers who performed
contract terminations. The guild had different sub-guilds like the bounty hunter division. The leaders and best members formed the so-called Elite Circle. Members were voted into the circle by their peers. Keluda was a member.
Sometime before the Battle of Yavin, The guild took a contract on the gunslinger Gallandro. However, this contract became the only known one they ever defaulted on after Gallandro personally killed half their Elite circle and assorted journeymen and apprentices.
While "Assassins" is spelled in a plural form (without an apostrophe) in The Weapons Master!, later comics The Third Law and Honor and Duty spell it in a possessive form (with an apostrophe). Presumably, they are still both the same group.