Mmmk it's been a while for me, haven't touched Mass Effect 2 in months, but I'll try to help.
That Krogan, release him, he's awesome, he's pretty much the "tank" of the team if you bring him with you on your missions, so release him from that tank (no pun intended). Click on the tank and you'll have a short conversation about it with EDI before you can take your decision about him.
The other teammates, you need to recruit or rather "find" them. You have been given the dossiers about them from the Illusive Man at the start of the game when you speak to him via hologram before you meet with Joker and see the SR-2 for the first time. So those dossiers have some details, but the location of each of the teammates is indicated on the Galaxy Map as missions, you can do those missions in any order you chose.
If you're on the Citadel then perhaps you've been there because you have a mission or you decided to go there yourself. Just open your Journal and see if there's anything to do in the Citadel. There's a Loyalty mission at the Citadel so maybe that's what you have to do (can't recall... I think it's Thane's Loyalty mission).
Overall, other than N7 Assignments (side quests basically) what you have to do is indeed building your team along with doing each of the teammates' respective Loyalty missions before the end-game moments (won't spoil anything). You will have if I recall something like two or three missions that you'll have to do when presented to them, mostly given by the Illusive Man from that conference room on the Normandy. So whenever EDI tells you that the Illusive Man wants to speak to you then expect to receive a mission that you will have to do at that point.
So hmmm... I hope it helps, I think that most of this is accurate enough. If you have more questions then I would recommend the Mass Effect Wiki site where you can read about all three game's missions in details and what they consist of, where to go, when to go where and who to talk to, etc.