The first sentence is usually used without will. Using it with will brings will closer to its sense of willingness rather than as an indicator of future time, and so it sounds like
The people who are willing (or show a desire) to attend the party tonight and demonstrate this desire by actually going to it will take part in the party tonight.
The second sentence is the same. It sounds like you do not have a girlfriend when you say the sentence, so you could mean
The girl who is willing to be my girlfriend will take part in my party.
or you could be predicting that some girl will become your girlfriend by tonight.
If you have a girlfriend, just use The girl who is my girlfriend will take part in my party.
In your third sentence will in will be found at 10pm comes across as a strong promise. You would probably not use is found at 10pm unless describing the situation after the girl was found; in this case the use of the present would be the historical present.