Hello, I am Dr RB:) The first thing is to determine if the inappropriate elimination is due to a medical reason or is truly a behavioral issue.
Excessive urination (especially in the presence of increased drinking) can indicate a number of conditions such as feline lower urinary tract disease (which mimics a urinary infection but due to inflammation within the bladder wall with no infection present), kidney disease, diabetes or hyperthyroidism. I would first be a great idea to ensure this particular cat has has current (within 3 months) exam, blood test (including complete blood count and blood chemistry) and urinalysis completed to rule out these causes.
When all of the above steps are taken and all medical causes ruled out, then it can be called truly a "behavioral issue." This can be due to a change in hierarchy between the cats, aggression or being aggressed toward, change in environment (new house, new schedule, new people, new pets, etc), stress from increased traffic to litter box area, not enough litter boxes or the undesired type of litter for this cat. First, for 5 cats, you should have a minimum of 6 litter boxes and litter boxes should be kept in low traffic areas in the house without relatively low noise level for distraction or stress. Not all cats prefer the same litter type; some inappropriate elimination can be solved by giving the cat a buffet of litter choices (such as clay litter, fine clumping litter, scented/unscented, paper litter, pine litter etc) until you find the right preference. Lastly, in the areas that he/she is urinating frequently, make sure you are cleaning the areas as well as surrounding walls/furniture with an enzymatic cleaner that is made specifically to break down the ammonia within the urine. Oftentimes, normal household cleaners (including bleach) do not do this and ammonia scent is left behind, not smelled by humans but still by cats, thus the repeated marking of the same area over and over..
I hope this advice is helpful to you. First, start with ruling out a medical cause to be safe and move to behavioral issue after that. Let me know if you have other questions or need clarification on any of this. Also, if my advice as helpful to you, please take the time to rate my response positively:)