You claim land by moving a unit over it and conquering it. It increases the tile's population to 1, which may be what you're thinking of.
Once we get 5 players, I'll go over the basics of playing the game, since the tutorial doesn't cover it all.
No, you claim land by having a owned tile adjacent to it, and then claiming it. You don't need troops in or adjacent to it (although you can't claim any land that has enemy troops in it).
You can also claim land by having a boat adjacent to it (which is the only way to claim other islands).
Having troops to start does make a pretty big tactical difference though, especially if they are stronger then spearmen.
Suggestions:
Add more iron sources.
Add rivers (which can be done by clicking on the "no river button"). You really want to add a significant amount for each portion of the map to speed up pop growth (which on a map this size is going to be massively important).
Give more mountains/hills to Islanders (for stone).
Give some close by sources of wood/stone to the Native Tribesmen.
Some bonus food on some of the coastal water tiles (otherwise there is almost literally no reason to claim any).
Give each starting player bonus pop to start (in addition to a few more tiles). With a map this large +10 might even be advisable. You may also want to give them some starting land in addition.
Do something with the top 5 tiles of the map. I would suggest removing them, but I'm not quite sure how you would go about that short of a program to edit the game code or redoing it all manually. You could also throw in some northern islands over there.
The easterners appear to have a pretty big advantage: They only have a single potential enemy, whereas everyone else starts with 2+ people that *can* attack them.
You also have to make a big decision.
A) You play with the default rules. This includes mages, endgame units, magical creatures, and some rare resources.
B) You play with the rules used in NQT's game, which I edited to take out all magical units and quite a few of the endgame stuff.
C) You play with a set of rules (also developed by me) that has some notable balance changes. Notably: Melee cavalry have speed 4, ranged cavalry are noticeably weaker (but get to keep speed 5), there are some speed 1 heavy infantry, pikemen/halberdiers get 50% better chargebreaker, and battering rams are removed and replaced by ballistas (25% siege, but are strong range 10 units).
D) You make your own set of rules, which while not very hard, seems a bit unlikley with how well you know the game.