Coalitions have a set amount of members, and so if the Coalition is full, nobody else will be able to join.

Please check the Coalitions menu in your game, as each coalition will be shown alongside a set of numbers. If you see, for example, '3/3' by your chosen Coalition, this means that it has three members, of a maximum permitted of three.

Remember that this can change depending on your map, with some maps permitting only two members, and others, up to five.