I may be wrong, but seeing as there are a lot of contract-based mechanics in this game, it would appear logical to assume that guild membership may also be contract based. If so, then one could assume that in this contract there may be terms of exclusive membership, for close-knit organisations which require entire devotion, and other non-exclusive organisation which would not bind oneself to its cause, for instance; a Baker's Guild which just provides a nationally respected standard quality for baking, organises events for like-minded bakers in guild halls and provides a hub for communication and integration among bakers to maximise profit.
To further the idea of a non-exclusive guild, one could imagine a scenario where an Information Broker's Guild could exist where secrecy is key and 'involvement' in other guilds is crucial for gaining items of valuable information. I guess this contract would be in their super-hidden section of their house! That is, if I'm correct in thinking they will be contract based. This would of course use non-exclusive terms of membership and enable membership in other guilds.
For realism, it would make sense when there is a crafting guild with some kind of monthly payment of coin so they can benefit from its convenience and such, which I guess could be a term of membership in said contract. For a more exclusive guild, I guess it would be a "In signing this document, you acknowledge that you will be a solider within [Military Guild] and that you may die or sustain serious injuries...blah...blah", just sign here so we can send you on suicide missions - sort of thing.
I hope you get my drift and I hope those who have vigorously scoured the Journals could correct me or shed some authenticity. I think this would make sense if we are talking about realism.
Another thought; perhaps different identities would play into someone being in and breaching the terms of two exclusive guilds?