As I’m pretty new I still don’t know how some things work here 
I just wanted to follow someone, went to his/her profile and saw a green “Follow back” button. Okay, there it was, now I know he/she is following me and I follow back 
On my own profile I can see, how many people are following me, but not who they are. Is there a way to see this information somewhere?
You should be able to see who. On the left side of your main profile, there are the categories ‘courses’, ‘mems’ and ‘mempals’, the former and final separated into ‘learning/teaching’ and ‘following/followers’. It should show under the relevant tab there, for example, that I am following you; kind of weird knowing that was probably my profile you were at, lol.
Great, thanks!
I knew one could see that under Mempals - but I forgot it again 
And so I tried to click on the number over “Followers”, which wasn’t a link
Why? I only went to your profile in the forum and saw the link to your Memrise account. That was the next question which was answered as I saw it. One only has to put the user’s name behind this link: https://www.memrise.com/user/
Only weird because your question must have been considered while you were there. I know I left the link on my community profile (and it is, as you say, as simple as putting username in the address bar, anyway).
You need some more people on your leaderboard… I’d say follow @tampora, but that’ll probably only depress you (one point three mil already this week — tamp’s on fire, as ever).
Don’t know, if I really need more people on my leaderboard 
I know it from Duolingo and this “leaderboard” isn’t something I usually look at very much.
I would rather join a group or something, but don’t know about groups here either
Groups here are mostly formed of several people taking the same course — they have a private leaderboard and, I suppose, can set up a group thread on the community if they want to chat outside the course. I’m not in any groups, so I can’t say much about what it’s like. They might do more things on the app but I wouldn’t know, being desktop only.
I have to say, I never looked at the Duo leaderboard either. I don’t even think I follow anyone on there. I do look at the Memrise board because it’s on the left of my main page and I scroll past it a dozen times a day; it’s a bit depressing when it’s empty, but I’m lucky enough to have several very active followers keeping me off the top of my own. It’s more like a personal encouragement board than anything (especially as following isn’t always reciprocal), rather than anything serious; must admit, something about the Duo board always makes me uncomfortable, though I can’t imagine why…
Just created a test group. Yeah, I only can invite people to the courses I’m learning.
And as I don’t know anyone… at least someone who learns Portuguese (Brazilian), hm… So it’s quite useless for me
Searched for groups in the support section and there it’s said: “the Group function is not available on the Memrise Mobile App at the moment”
As I’m also using the deskop version
Yeah, now it hasn’t any function, only to see the daily/weekly… progress. Long time ago there was the activity stream (on ones profile), which they removed. So I only follow people, who are really helpful or I had one or more conversations with.
If one doesn’t look at one’s own profile, one doesn’t even realize that one has a follower 
Maybe that’s one of the reasons?