I'm the kind of person where if I'm in a group text chat whether it's work related or not, I never add anyone's number to my contacts without their permission regardless of how long I've known them. It may sound like I'm just being weird but I do know that it can be seen as creepy if you just add their number, they may not want you have it and may be perceived as strange if they found out their number is in your contacts. I only add people's numbers if they text me one-on-one first or if they specifically tell me that I can add their contact info into my phone, other people have found it shocking that I don't add people's numbers right away but I just don't feel comfortable without their permission. That goes for asking for other people's contact info too which I've talked about before, I never ask unless I absolutely have to which I absolutely hate doing but otherwise I let others take the initiative or even give them my number first and let them decide what to do.