If youre looking for something more than just a group then I will recommend you join Reddit. There are lots of Bitcoin and altcoins enthusiasts there discussing on very interesting topics. You will also be getting updated there regularly there with lots of topics to engage in. If youre looking for just a group that also has a lot to offer, then you can go for Telegram. But in Telegram you wont be able to join any group without having the link to do so. Before you join a group, someone will have to give you the link to do so.
People having admin rights in the telegram channel and group can only able to share the joining link of the concern group. Others will not be able to do that. As you telegram is the best place for the project mates to discuss about it.
There are some other option also recently used in wide range mate. I suggest to use the Discord or slake channels for the communication medium.
If there any new implementation such as groups option here in this forum means also here all bitcoiners will love it and go towards it. No more we need the help of the social network mediums.
other users can also share thier links thru telegram because some other channel do allow it , not unless you are spamming your links then that are not tolerated by them. And what is that slake are you talking about? do you mean slack? if it is , then slack are now rarely used as a comunication in most bounties because slack has recently reported to be prone in scams and hacks due to the fact that the slack security is pretty poor. discord isnt also a good platform because it is to complex and not a user friendly app.