If a user is assigned to multiple campaigns, then it has to select the campaigns in which it wants to login during its logon to the system.
"Default Working" Tab in a Campaign lets you select the Default Campaign for the selected users. These users will not be prompted to select the campaign and will be logged on to this campaign by default.
Figure: Set Default Working
You just have to select "Default Working" checkboxes for the users which you want to logon in this campaign by default. Click "Apply" to apply this change.