The Membership Act defines the conditions of membership for the Union. Additionally, it defines the process by which new members may join the Union.

The conditions generally include things like having complete control over their home system, being of sufficient size, not being at war and being economically self-sufficient.

When a member wishes to join, they become a Candidate World. A general investigation is done to determine that all information provided by the new member is correct. If so, the new member becomes a full member when the investigation is concluded.

Before being declared a full member, a prospective member may be challenged if a world believes the new member is unsuited. This can be done for example if the world is at war with a member of the Union, if it does not qualify as sovereign or if its population is too small.

If challenged, a separate investigative committee is sent to the planet to determine whether or not the new member qualifies. This investigation must return satisfactory results before the general investigation can be concluded.

The candidate period is used by the Storyteller to check if all the relevant stats are correct, if no mistakes were made, etc.