Sourced from https://tributedao.com/docs/contracts/core/dao-registry/

What does "membership" mean in Tribute / Open Law?

Defined directly via the Members struct. All members have some form of voting right.

Background: the DAO Registry contract is the identity of the DAO. This is the contract address that every adapter usually interacts with. The DaoRegistry.sol contract tracks the state of the DAO for 1) Adapter and Extension access, 2) State of Proposals, 3) Membership status. If an Adapter needs to access the DAO Registry, it must be registered to the DAO with the correct access flags.

How to query membership?

DaoRegistry (and other adapters too) inherits from MemberGuard. This parent smart contract

has the function isActiveMember that will let you know if you are an active member or not.