The What exactly is a guild mark? question made me wonder about this aswell.
Can a guild mark be forcefully applied on somebody? Or is it a process of mutual acceptance. Or maybe even like with the celestial spirits, a contract?
Gajeel, a former Phantom Lord member, joined Fairy Tail after the defeat of his former guild. Also,
Moved by Makarov's mercy and compassion, Gajeel accepts his offer.
Juvia, like Gajeel, was a former Phantom Lord member. She joined Fairy Tail after the defeat of her former guild.
Juvia goes to the newly renovated Fairy Tail guildhall to meet with the guild's master, Makarov Dreyar, now prepared to ask him for guild membership.
Lucy Also joins the guild on her own accord.
From the above examples, we can see that each character of the story joins the guild upon the mutual agreement of the Guild Master and him/herself.
Yukino, upon her loss she was forced to strip and asked to erase her guild mark, thus banishing her from the guild. She was very disappointed with this.
After failing to win, Yukino faces the anger of Sabertooth's Guild Master, Jiemma. Displeased by the failure of Team Sabertooth on the second day, Jiemma throws grapes at Yukino's head and forces her to strip, telling her to erase her guild mark, thus banishing her from the guild. She is later seen standing outside Honeybone Lodge, the current lodging of the Fairy Tail Guild for the duration of the games.
As he informs the Thunder God Tribe of his expulsion, Bickslow tries to convince him otherwise, but Laxus tells him not to take action. Laxus watches the Fantasia parade before he leaves and realizes that everyone, including his grandfather whom he despised, is still looking out for him. Laxus leaves the parade realizing what a fool he was, smiling but shedding tears of regret.
A member is excommunicated from the guild at the discretion of the Guild Master as we can see in case of Laxus and Yukino.
Also, as Sp0t mentions, no guild would forcefully apply on anyone their guild emblem to reveal their magic. When someone joins the guild, he/she is privy to the working of the guild and also some secrets. Why would a guild master force someone to join and out these critical information at risk? As we have seen that Gajeel was used to counter spy on Ivan which was devastating to the Raven Tail Guild.
So I would say that a member joins a guild with the mutual agreement of the Guild Master and him/herself.
P.S: there is always a possibility where a character is blackmailed to join a guild, but if we blind side the "blackmail" part we can say that he agreed to join the guild. The agreement may not be completely mutual, but nevertheless it's mutual (ugh... I know it's complex). But I doubt that such an agreement can be beneficial to the guild.