So there are 7 ingame companions in total. But at one point You can only have 5.
So, if I decide to remove some of them from my team to make way for others, are they lost forever? Or can they be added to the team once more?

You can switch them around, but some actions might make them leave permanently such as doing stuff they don't like. So just make sure to keep everyone happy.

And where do they go 'to wait' once I decide to temporarily remove them from the group?

They'll wait where you leave them. Keep in mind however that there are locations which you'll visit only once. It's not a good idea to leave your companions in Modron's Maze and in locations that you'll visit after Sigil (if you want to add them to your party once again that is).

So technically speaking I should leave them only in Sigil? That means, I will leave that Sigil and return to it later on, right?

You'll return to Sigil after a while, but by then the companions you leave in Sigil will be heavily underleveled. It's good to decide about the final composition of your party before you leave Sigil. Keep in mind however that one of the companions won't be available until you leave Sigil.

I know - Vhailor is the one. And since I know od of the b******* later in the game, I think I know which companion to ditch, given that I am Lawful Good character at the moment.