It is a real struggle to open up to your colleagues and feel comfortable around them if you are an introvert. It is extremely difficult to let someone into your world and share your thoughts with them, without thinking of what will they think of you. If you are going through the adaptation to a new team, I know your struggle.
The best thing you can do is give yourself time. Do not feel like you have to be social just for the sake of seeming to be social, no. Being “slow to warm up” is normal and there are many people like you, so the important thing you need is time. It can be weeks, can be months, but step by step you will start to include people around you at work in your comfort zone.
Another good thing you can do, is start small: try to communicate with others in a smaller group, it is much easier to get to know a person and feel comfortable around him/her if you are in a group of 2-3 people. So split the big group and start communication with people one by one, afterwards you will feel comfortable in that original big group.
Also, remember to accept people as who they are, and they will accept you. Patience and understanding are the keys to a great environment inside the team. And be open about your struggles with your colleagues. You don’t have to inform everyone about your feelings, but find a person with whom you feel most comfortable with and tell him/her about your challenges. You will feel more confident when you know, there is at least 1 person in a team who knows about the difficult adaptation process you are going through right now.
I know how difficult it is to adapt to a new work team if you are an introvert, but take your time, take it slowly, accept people as they are, and eventually it will become your comfort zone.
May the force be with you!