If someone asks you to be on the team, you can tell her:
"I have a lot of strong ladies on the team, so I can't guarantee that you would get a chance to play any matches." If she still want to join your team under that circumstance, then let her sign up, since obviously it's not about the tennis for her - it's about her need to feel included.
If someone whines about not being invited, tell her:
"I have a lot of strong ladies on the team, and I didn't think it would be fair to you to invite you if it was unlikely that you'd get a chance to play a match."
The only way these exchanges might become awkward would be if the player had an overly inflated view of her own skill level.
BLX Blade 98. 13.58 oz., 12.55", 366 SW. 16x20 (outer mains skipped)
Pre-Stretched Ashaway Kevlar 18g/ZX Monogut Natural 16g, 66/46 lbs