If you have a joint tenancy agreement, all the tenants have exactly the same rights. You are all equally responsible for paying the rent and keeping to the terms of your agreement . If one tenant is not paying the rent or causing other problems you could end up having to pay her/his share, or any other costs. Your landlord may be entitled to keep the deposit if there is any rent owing or damage to the property at the end of the tenancy.
If one joint tenant ends the tenancy, everyone will have to leave unless those that want to stay can negotiate a new tenancy with the landlord. This is a complicated area and you should get advice.
Similarly, the landlord cannot evict one joint tenant without evicting all of you. However, s/he may decide to offer a new tenancy to the remaining tenants once the original tenancy has ended. Talk to your landlord about this if you want to stay.
If the tenants have disagreements, they are responsible for sorting them out between themselves. Only in extreme cases will the landlord or anyone else get involved.