Men can have a tough time dealing with a girlfriend who is very possessive and jealous about the relationship he shares with his male and female friends. Her possessiveness could be overwhelming, irrational, illogical and some times inexplicable.
A girlfriend with this tendency needs to be handled delicately so as to not to disturb the relationship you have with her and also the relationship you have with your friends.
She may feel jealous for variety of reasons, which may be unfair or fair. Getting to know her reasons are important so that you can set things right. Be patient, and alert to her comments, feelings.
The reasons could be anything like spending more time with friends, sharing intimate secrets with them (especially with girls), having long conversations with them, not including your girlfriend in your social circle, keeping secrets or hiding some things from her.
Marking a private space in each relationship is important. Explain to her what your privacy needs are. Draw it out clearly. For example, tell her if you do not like her going through your sms, call list, if you need to spend some time with friends too etc. Similarly, ask your girlfriend about her privacy needs and do not violate it.
Decide about the level of disclosure you want to share with each other. Being in a relationship does not mean that you have to disclose all your plans, thoughts, secrets of your friends etc with your partner. Neither do you have to give each other a description of everything you do in a day.
Be honest and straightforward in the relationship. Do not pay games with each other. Having a stealthy behavior, suddenly avoiding the partner, suddenly keeping your mobile switched off or abrupt change is sure to cause suspicion in the mind of the girl.
Keep her updated about important developments in your life.
Introduce her to your circle of friends so that her sense of threat, insecurity, fear, and her feeling that she is an outsider will reduce.