Officially, there is no precise limit to the number of contacts in a group. ( I assume by "members" that you mean contacts.) But I was advised by a Google staffer, on a rare occasion when I actually was able to talk live to a Google representative (the Age of Miracles is not yet over) that it is a good idea to hold the number of contacts in a group to less than 50. So, I set a limit for myself of no more than 45 contacts in a group. But be advised that this is all speculative, theoretical, unverified.
Definitely official is that Google will not allow you to use an individual gmail account to send email to more than 500 contacts (individual and individual within group total)
within a 24-hour period. Unless you catch the system when it is not functioning for some reason, an attempt to do more than that will be blocked. You will be told you are over the limit, go away, and don't come back until later.
If you want to beat that daily limit, the way to do it is to establish another separate account, or several accounts if you want. Each account will then give you another 500 contacts for each 24-hour period. The one trouble is that you cannot use the contacts list in the address book of one account for use in another account. You have to set up a whole new address book for each separate account.
Caution: If you are using your Gmail account(s) for business purposes, take it easy. You can do that to a quite significant extent and get away with it, but if you overdo it you risk the chance of Google's shutting you down. I do not say that will happen, and I know there are Gmail users making extensive use of accounts that are purportedly for personal use to do extensive promotion of their businesses, and are getting away with it. So, where is the line and when do you know if you have crossed it? I guess you will not know until Google tells you.
Hoping this helps, Wolfeman (aka Burton H. Wolfe)