As WayneLigon said...
Guilds will have own laws and rules (what magics are castable, when and where)
Guilds provide sharing of knowledge, being libraries and trading post for wizards
Guilds also can provide papers - things like right to cast, proof of insurance, familiar license
Guild membership can also be a tax loophole (local taxes at a reduced rate or write-off)
Guilds provide work - guilds can assign members to task, such as: education, adventures, item building, assignment to post
As far as sorcerer goes, that is up to you and you have to ask the question; how do the wizard of your campiagn view the sorcerers? If they don't care one way or another they will let them join. If they dislike, well thi scould be fun as you could have two guilds, the free guilds (all can join) to the Wizard's Guilds (only wizards). Now you can think of their relationship to each other as a power groups in your campaign.
Guilds will have own laws and rules (what magics are castable, when and where)
Guilds provide sharing of knowledge, being libraries and trading post for wizards
Guilds also can provide papers - things like right to cast, proof of insurance, familiar license
Guild membership can also be a tax loophole (local taxes at a reduced rate or write-off)
Guilds provide work - guilds can assign members to task, such as: education, adventures, item building, assignment to post
As far as sorcerer goes, that is up to you and you have to ask the question; how do the wizard of your campiagn view the sorcerers? If they don't care one way or another they will let them join. If they dislike, well thi scould be fun as you could have two guilds, the free guilds (all can join) to the Wizard's Guilds (only wizards). Now you can think of their relationship to each other as a power groups in your campaign.