First off, don't let her come into your room. That should be a no-no. The aim is to resettle her in her own room. does she have a nightlight? If not, maybe invest in one.
Go in, settle her, but make her understand she must remain in her own room. Stay with her for a bit. Leave for a few minutes, go back in to check on her, leave again. If she gets out of bed, put her back in again and resettle her.
Have a consistant bedtime routine - eg bath, supper, story, cuddle, milk, bed..so she knows whats coming.
Invest in some monster repellent (yes you can get it!)
I'm afraid there's no way of sorting it that doesn't leave you knackered in the short term (sorry) but it takes time to sort it out.
Maybe a star chart - star for each night she remains in her own room - X number of stars in a line (ie no gap where she came into bed with you one night!) gets
Have you been burgled btw? or any of your relatives, or friends? If so, she could be anxious about that.