This is an interesting topic and deserves more detail about league play rather than being restricted to ratings.
In the UK (and in my experience)...
There are very few public courts, and so if you are in any way serious about tennis you will almost certainly be a member of a club. Clubs will usually have teams entered in the league, the number of teams being determined by the size of the club. To my knowledge, there is no maximum limit per club. Leagues are organised by county (essentially our equivalent of states, but geographically much smaller), and are exclusively doubles. In the summer it is usually divided into mens and womens, and in the winter it is usually a mixed league. For each match there are two couples, and they will play both of the opposing pairs in a best of three sets format. Often a championship tie-break is used instead of a third set, especially during winter when floodlights are required.
Ratings are not given for doubles play, and there is no such thing as a '4.5 team'. The league system is much the same as our other sports, where teams are divided into linear divisions. At the end of each season, the top 2 or so teams in each division are promoted to the division above, taking the place of those who had finished bottom and were hence relegated.
Players are affiliated with their club, and cannot play for another club in the same league during that season. However, within the club they are not firmly allocated to any team. The rules are usually that once you have played 2 matches for a particular team, you cannot 'play down'. For example, if a club has 5 teams (A, B, C, D and E), once a player has played twice for the C team they cannot represent the D or E team that season. There is an element of sandbagging going on, largely because successful teams will attract more players to the club. The most common time for this to occur are at the start of the season, where a player may play 2 games for the C team to ensure they get a good start before playing in the B team for the remainder of the season.
The rating system here is purely based on singles play, which is restricted to either tournaments or matchplay. The vast majority of adults do not have a rating, and those who do will most likely have played fairly competitively as a junior.